Who has baseball's best rotation? With so many clubs missing their biggest arms and relying on wild-card options, there really isn't an obvious answer.

The Phillies still have Zack Wheeler and Aaron Nola at full health; the D-backs have a pair of healthy front-end starters in Zac Gallen and Merrill Kelly. Then there are the Dodgers, the Orioles, the Yankees -- point being, there are a lot of defensible answers to that question.

But if we look at run prevention in its most basic form -- straight ERA, no need to overcomplicate things -- the answer is none of the above. Leading the league through Thursday, far and away, were the Red Sox (1.82) in first and the Royals (2.39) in second.

Raise your hand if you saw that coming. (We sure didn’t.)

A year ago, Red Sox starters ranked 22nd in baseball with a 4.68 ERA; their counterparts in Kansas City ended last season 27th at 5.12.

Boston, coping with a depleted staff for several seasons now, made just one addition to its Major League rotation over the offseason in the form of Lucas Giolito. (He sustained an elbow injury during Spring Training and is likely out for the season.) Kansas City’s farm system had struggled for years to develop Major League-caliber pitching prospects, leading the club to look externally for some reinforcements this past offseason.

So far, though, both rotations are excelling. It's intriguing, no doubt, even if we’re talking about roughly 12% of a long, grueling season.

The real question is: How much can either team count on this continuing? Is the early success something sustainable, or a product of luck, good matchups and small sample sizes? There is no definitive answer here, but we can look at what’s changed for a few of the major difference makers for both rotations and try to come to a verdict on which will have the most staying power. (All stats are through Wednesday’s games.)

• Can these 7 teams keep up their early success?

Red Sox

Kutter Crawford's four-seamer

Crawford has, thus far, been Boston's real prize -- his 0.42 ERA through his first four starts is the lowest for a Red Sox starter since Roger Clemens posted an 0.28 ERA in his first four of 1991. Things started to come together for Crawford last year, when he found a groove with his four-seam fastball -- opponents hit .163 against it with a 36.5% strikeout rate (fourth in MLB, minimum 150 plate appearances ending on four-seamers). This year, they're working on a 1-for-22 (.045) line against that pitch. How utterly unprepared they've been for that pitch may have something to do with another shift; Crawford has used his sweeper and cutter almost as much, each a little under 30% of the time.

Tanner Houck, pounding the zone

How about Houck’s gem on Wednesday night against the Guardians? His three-hit, no-walk, nine-strikeout, 94-pitch shutout was the first “Maddux” thrown by a Red Sox pitcher since 2014 and took just 1 hour and 49 minutes to complete. That dominant performance was indicative of Houck’s big change: His walk rate has dropped from 8.9% in both of the past two seasons to just 1.9%, ranking in the 96th percentile of MLB pitchers. Houck’s 56.7% zone rate ranks among the MLB leaders, and his wipeout slider (also his most-used pitch) is at 58.0%, up from 42.7% last year. Now, he can actually pitch off of that slider -- especially in games when neither his sinker nor his splitter are working as well.

Brayan Bello's retooled repertoire

The Red Sox emphasized their faith in Bello with a six-year extension in March, and although his 3.92 ERA is one of those pulling up the team's average, there's been a lot to like about his first four outings. Most notably, he's yet to show his four-seamer. After his Opening Day start, Bello told The Boston Globe's Julian McWilliams that he doesn't intend to drop the pitch altogether, but rather that he's hoping to use it more strategically in the future -- probably for the best, given the trouble it gave him last year (.310 BA, .646 SLG). Instead, Bello is using his sinker, changeup and slider more, with opponents going just 2-for-22 against the change.

Royals

Cole Ragans' upside

Still a fairly under-the-radar figure, Ragans got some preseason votes to win the American League Cy Young Award. It's fair to ask what that was all about, because for how well he pitched down the stretch for the Royals in 2023 (2.64 ERA, 11.2 K/9 rate after being acquired from the Rangers), that was still only 12 starts. But so far in 2024, he’s backing up that faith (1.93 ERA with an identical K/9).

Ragans' best pitch is, far and away, his changeup, which held hitters to a .192 slugging percentage in 2023 and has a 51.1% whiff rate (23 misses/45 swings) in the early going. His four-seamer went from averaging about 92 mph with above-average arm-side movement in '22 to well over 96 mph in '23 without compromising any of that run. He added a slider on the fly last season -- hitters went 9-for-50 (.180) with one extra-base hit against it. That only covers three of his five pitches (he also throws a cutter and a curveball, both almost exclusively to righties), all of which he comfortably uses over 10% of the time.

• How Cole Ragans became a budding ace for the Royals

Brady Singer’s key adjustment

Not to belabor our point about the Royals' ability to develop pitching, but Singer is, by FanGraphs’ version of WAR, the most successful pitcher Kansas City has drafted and kept in-house since Danny Duffy. Problem: He's also a sinker-slider guy who, until 2024, basically only threw sinkers and sliders. Worse yet, his sinker wasn’t playing very well at the Major League level. Hitters had a .339 AVG and .555 SLG against it in '23; he, consequently, had a 5.52 ERA and 1.45 WHIP in 2023. This season, though, Singer is leaning more on his slider (46% usage) while significantly cutting down on his sinker usage in favor of a previously little-used four-seamer. That mix is working for him, with the slider, sinker and four-seamer all allowing a sub-.200 opponent average. Singer (1.54 ERA) has not allowed more than two runs in any of his first four outings.

KC’s canny veteran additions

Another huge difference between the 2023 and '24 Royals rotations? The veteran acquisitions. Kansas City essentially swapped out Jordan Lyles (6.28 ERA, now in the bullpen) and Zack Greinke (5.06, now a free agent) for Seth Lugo (1.05) and Michael Wacha (3.75), both signed to relatively modest free-agent deals in the offseason.

The Padres converted Lugo, a staple of the Mets' bullpen from 2018-22, into a full-time starter in '23. Given how long he'd spent being used primarily as a reliever, that experiment was remarkably successful; he made 26 starts, pitching to a 3.57 ERA with 140 strikeouts in 146 1/3 innings. And, assuming he stays healthy, there's reason to believe this experiment has legs, given Lugo's ever-evolving repertoire and willingness to pitch to contact. It's certainly working so far.

Issues with Wacha’s right shoulder cut off a strong start to his career with the Cardinals, and he largely fell off the map in the late 2010s before re-emerging in '22 as an unexpected lifesaver for the Red Sox. Since then, in his time in Boston, San Diego and Kansas City, he's thrown 285 2/3 innings while maintaining a 3.31 ERA. His changeup in particular has lent legitimacy to his comeback -- it's ranked in the top five by run value since the start of the 2022 season, holding hitters to a .193 average.

The verdict

Either of these rotations is liable to come crashing down to Earth (which, to be fair, happens frequently after hot starts). But the Red Sox may have a tougher time keeping their success going given their health issues. They already lost Giolito prior to Opening Day, Nick Pivetta (0.82 ERA in two starts) soon joined him with a right elbow flexor strain, and now Garrett Whitlock (1.96 ERA in four starts) has been placed on the IL as well, with a left oblique strain. While Pivetta and Whitlock could be back relatively soon, Boston does not have a lot of quality depth available to patch those holes.

Given the cobbled-together nature of the Red Sox pitching staff before Whitlock got hurt, this could get ugly. The Royals, a little less injury-plagued and relying more heavily on adjustments within their control -- Singer can throw as many sliders as he wants, after all -- would seem to have an edge.