A month into the season, several of baseball's best starters are off to what look like slow starts, and in some cases, that's all they are: Slow starts. Maybe it's poor defense behind them, or one bad game that inflates their ERA for months to come, or something else that makes the surface level numbers look poor while the underlying skills haven't changed. But in other cases, every bit of that poor start -- and more -- has been earned. Knowing the difference is crucial.

Every 6.00 ERA isn't created equally, is the point. But how can we tell the difference? We'll try to dig a little deeper to figure out how some of baseball's most notable names ended up with these numbers.

Let's look just at long-time stars -- not you, yet, Pablo López, even though you've out-pitched that 4.78 ERA -- with ERA marks north of 4.00. Let's pick 10 of the most interesting ones.

For each starter, we'll list ERA, Fielding Independent Pitching (or FIP -- which attempts to eliminate fielding and ballpark factors to take strikeouts, walks and home runs, and put them on an ERA-like scale, with this year's average being 4.40) and a Statcast ERA equivalent based on Expected Weighted On-Base Average (or xwOBA, a Statcast metric that accounts for quality of contact and amount of contact).

Finally, we'll break them into three sections (not worried at all, a little worried, and very worried). There are different ways to have a "bad start."

1) Don't worry, it'll be fine. Probably.

Max Scherzer (4.12 ERA, 2.25 FIP, 3.09 Statcast ERA)
Scherzer's ERA, if it held, would be the highest he'd had since way back in 2011, before his breakout from "interesting young talent" to "probably a Hall of Famer." It's fair to say that he's more than earned the benefit of the doubt here, and that's true, but it's also true about a lot of these pitchers. So what makes things different for Scherzer?

Well, he's still missing bats, for one. He's tied with Gerrit Cole for the most strikeouts in baseball (54), he's striking out nearly 11 batters for every walk -- a 10.8 K/BB that's also the best in baseball -- and he's actually managed to get the ball on the ground more, with a 46 percent grounder rate that would be a career best.

That's why those FIP and xwOBA marks (where Scherzer is still a top-15 starter) remain so stellar. That the ERA hasn't kept pace is due to a poor outing in Miami (six earned runs), poor Washington defense, and a lack of help from his relievers. (Like on April 7, when he left with two men on, only for Matt Grace to immediately allow a single and a home run.)

He's fine. It will be fine.

Gerrit Cole (4.71 ERA, 3.30 FIP, 3.28 Statcast ERA)
Speaking of Cole, there's not much to worry about here, either. As we noted, he's collecting plenty of strikeouts, and he's had a few truly dominating outings, like when he struck out 10 Rays without a walk on March 29 or 11 Mariners without a walk on April 14.

The problem here, if there is one, comes down to one bad game. On April 20 at Texas, he allowed nine runs (eight earned) in 4 1/3 innings, including five in the first. This came amid concerns over pitch-tipping, and he was much better on Thursday, when he whiffed 10 and allowed only two runs in a 2-1 loss to Cleveland. Take out that one bad start, and Cole's ERA would be 3.09. Nothing to see here.

Carlos Carrasco (5.86 ERA, 2.92 FIP, 3.98 Statcast ERA)
This is already getting better. Carrasco had two tough starts in his first three games, waking up on April 13 with a 12.60 ERA. He's allowed four earned runs in his last three starts, and even that requires further investigation. He allowed zero runs in seven innings in Seattle on April 17, then threw four more scoreless innings on April 23 before leaving early due to an injury. On Sunday night in Houston, he took a shutout into the seventh inning before the Astros touched him for four.

Carrasco has jumped his already strong strikeout rate from 29.5 percent to 33.5 percent. He's allowed the highest BABIP in baseball, which almost certainly cannot sustain. He'll be fine. We're already seeing him be fine.

Noah Syndergaard (6.35 ERA, 3.72 FIP, 3.30 Statcast ERA)
It feels like a long time since the breakout 2016 that got a 23-year-old Syndergaard not only some down-ballot Cy Young support, but a few back-end MVP Award votes, too. In parts of three seasons since then, he's thrown only 218 2/3 innings with a cumulative 3.54 ERA. The talent still shines through, and the velocity remains, but it's been years since he's been healthy, available and productive at the same time for an extended run.

So when you see that he's going to end April with an ERA north of 6, you can understand the concern, but there's plenty to be optimistic about here. He still has the elite velocity, with his 97.5 mph fastball atop the starting pitcher leaderboards. His strikeout rate is up. His walk rate remains unchanged. His .283 Expected wOBA is 16th among starters, and as good or better than Trevor Bauer (1.99 ERA) and Joe Musgrove (2.06 ERA).

In fact, among all starters with at least 100 batters faced, no one has a larger gap between expected wOBA (.283) and actual (.355) than Syndergaard does.

All of those things suggest a pitcher who is throwing better than that 6.35 ERA tells you, so what is happening here? It's a little about the ball finding grass, because Syndergaard is fourth on the aforementioned BABIP list. It's a little about home runs, since he's allowed five already, after just nine last year. It might be a little about his reportedly being sick during a April 21 start where the Cardinals scored six times. It might be something to do with his secondary pitches, because he's throwing his four-seam fastball far more than he did last year and he's saying things like "I don't have any trust in my slider or curveball" and that "every time I get a new baseball out there it feels like I'm holding an ice cube."

It might, also, be nothing. Syndergaard is missing bats reasonably well (64th percentile) and avoiding hard-hit contact (86th percentile).

2) It's OK to worry a little.

Jacob deGrom (4.85 ERA, 3.71 FIP, 3.52 Statcast ERA)
After allowing five runs in four innings on Saturday night, deGrom offered an honest assessment of his evening:

"To go out there and do that," deGrom said, "it's embarrassing."

It hasn't been great recently, because in his last three starts, he's allowed 14 runs in 13 innings, all around a brief trip to the injured list due to elbow soreness. But remember, each of those three starts featured a weather delay to begin the game ...

April 9 -- 25 minutes
April 14 -- 27 minutes
April 26 -- two hours, 42 minutes

... and that his 2019 got off to an incredibly strong start, as he struck out 24 against only two walks in 13 shutout innings in Washington and Miami. deGrom's 2018 was so good that it was never likely that he was going to be able to repeat it, but his first two starts looked as good as ever, and his last three have come with more than a little interruption of his usual routine. (Not to mention, facing the shockingly dangerous Twins lineup.)

If not for the elbow concern, we'd put deGrom in the "don't worry" category. (His velocity isn't down, it's actually up, though it's interesting to see that he's using his four-seamer a lot more as his slider has been pounded early, suggesting a lack of confidence in one of his best secondary pitches.) It's not time to panic. It is time to take this seriously.

Chris Sale (6.30 ERA, 5.23 FIP, 5.54 Statcast ERA)
You know all about the Sale story right now, and it looks like this, with each start becoming an unofficial velocity watch across Greater New England. It's not great: The Red Sox have lost each of the six games he's started.

Maybe it's not quite as bad as all of this. We seem to go through Sale velocity questions early each year -- remember The Great Panic of 2016? -- and he did hit 97.5 mph a few times on April 16, and on April 23, he struck out 10 Tigers. Then again, it's more than just about the fastball, isn't it? His whiff rate on his slider is down from 44 percent to 29 percent. His changeup slugging percentage allowed is up from .329 to .538.

Whatever is happening here, it's real, and it's troubling. It's been nearly 30 years since a Red Sox pitcher had a month like this:

Corey Kluber (5.81 ERA, 4.20 FIP, 4.63 Statcast ERA)
Consider this more an extension of some slight warning signs buried within an otherwise typically outstanding 2018 season. While he won 20 games, put up a 2.89 ERA, and finished third in the Cy Young voting, his strikeout rate dropped from an elite 34.1 percent to a merely strong 26.4 percent, and his velocity continued its drop from his 2013-14 peak.

He's never been a traditional flamethrower, of course, so the velocity drop by itself isn't as concerning as the fact that his curveball whiff rate has dropped from 50.6 percent in 2016 to 49 in '17 -- still great -- to 37 percent in '18, a mark he's repeating this year. Overall, his strikeout rate has dropped again, to a merely average 23.8 percent in '19.

Perhaps more concerning: In each year since Kluber became a regular starter in 2014, his walk rate has hovered between 4 and 6 percent of batters. So far in 2019, it's 10.2 percent. It would be the highest of any month of his career. It's difficult to write that off.

Kyle Hendricks (5.33 ERA, 3.73 FIP, 5.05 Statcast ERA)
Kris Bryant doesn't think there's any reason to worry. "He's been a stud for us," Bryant said after Hendricks allowed seven earned runs in Arizona over the weekend. "You kind of know what to expect from him, so there's no reason to worry."

He's probably right. Prior to that, Hendricks had a 3.54 ERA in his first four starts. He'd just whiffed 11 D-backs over seven scoreless innings in his previous start. We're generally trying not to care about one lousy game. But the minor concern here is just how hittable Hendricks has been. In his entire career, he's allowed 10 hits or more four times, and two of them have come in his first five starts of 2019. (He didn't do it at all in '16 or '17, and just once in '18.)

Put another way, compare his last three years. In 2017, he allowed a .244 average, and a .375 slugging. Last year, it was .247 and .391. So far this year? .339 and .486. For a pitcher who is never going to rack up the strikeouts, this is an issue, even if we can't pinpoint the cause.

Aaron Nola (5.68 ERA, 5.41 FIP, 5.06 Statcast ERA)
You name it, and it seems bad. Strikeout rate? Down from 27 percent to 22.7. Walk rate? Up from 7 percent to a career-high 9.2 percent. Home run rate? Up from 0.72 per nine innings last year to 1.99 per nine this year. Hard-hit rate? Up from 31 percent to 45.7 percent. This is all bad. This is why the sky-high ERA isn't a fluke or a blip, it's the result of actual poor performance.

But what's far less clear is why this is happening. His velocity isn't meaningfully down, and there's not much in his pitch movement profile that suggests something is wrong. That's both good -- maybe nothing is irreparably broken -- and bad -- how to fix something if we don't know what it is?

His percentage of pitches in the zone is down, from 48 to 41 percent. Hitters are chasing fewer pitches outside the zone, from 35 to 30 percent, but they're also going after fewer pitches in the zone, from 60 to 54 percent. There's not one good reason here. That's what makes it so frustrating.

3) Sound the alarms.

Yu Darvish (5.02 ERA, 6.62 FIP, 5.36 Statcast ERA)
After a disastrous Cubs debut in 2018 (only eight games, due to an elbow injury) in the first year of a six-year, $126 million contract, Darvish badly needed to get off to a better start in 2019. He's shown flashes, like when he allowed only one run to Arizona in six innings on Saturday, but there's real concern here. His strikeout rate of 25.4 percent would be a career-low. His walk rate of 16.9 percent would be a career high. (It would, in fact, not be that far off the 19.6 percent walk rate that got Tyler Chatwood in such trouble last year.)

If the focus is on health, it's been better news; Darvish touched 98.7 mph this month. Still, the command issues are seriously concerning.

Miles Mikolas (5.29 ERA, 6.05 FIP, 5.02 Statcast ERA)
It's here, more than any other pitcher on this list, that we point out how early it is. Mikolas threw 200 2/3 outstanding innings last year, and this year's 34 disappointing ones don't erase them. But because he worked without much of a safety net -- last year's 18 percent strikeout rate was in only the 21st percentile, or worse than nearly 80 percent of pitchers -- any decline there was going to cause a serious problem.

So far, that's what has happened. Mikolas hasn't struck out more than four in any start. His 13.1 percent strikeout rate is the second-lowest of any qualified starter, while his walk rate and hard-hit rate each have crept up slightly. Plus, after allowing only 16 home runs in those 200 2/3 innings last year, he's already given up eight.

There's no obvious reason for this that we can find, and so no reason he couldn't turn it all right around. It's just that this 5.29 ERA isn't "bad luck" or "one bad game," like some of the others. It's earned.