There's no way to describe the start to Jackie Bradley Jr.'s season other than "disappointing," so we won't.
Headed into Wednesday's game, Bradley is hitting a mere .200/.277/.360, an overall performance that's among the 20 worst of the 228 hitters with as many plate appearances as he has. Two years ago, his line was 21 percentage points better than the Major League average. Last year, it was 18 points better. This year, it's 35 points worse. Of the dozen Red Sox hitters with 50 plate appearances, Bradley has been the least effective.
That's bad; no way around it. So why are we so optimistic about a rebound? It's partially because we've seen him be a lot better than this in the past, but mostly because all of the data is pointing in exactly the right direction -- even if the results haven't quite followed yet.
It basically comes down to this: Bradley is hitting the ball too hard not to be getting more production out of it.

We can say that by looking at Bradley's hard-hit percentage, which we've defined as above 95 mph, largely because that's where the data has shown to be a clear dividing line for success. In 2016-17, batters have hit .541 with a 1.074 slugging percentage when they hit the ball 95 mph or harder, and just .218 with a .255 slugging when they hit the ball 94 mph or less.
It's good to hit it hard, and it's good to do so often. So far this year, 383 hitters have put at least 25 batted balls in play. Bradley has put 49.3 percent of his batted balls in play with an exit velocity of at least 95 mph, and that's the 20th best in baseball, which is outstanding. It's better than Nelson Cruz (48 percent) or Freddie Freeman (47.1 percent) or Eric Thames (44 percent). It's better than Bradley himself in either of the past two seasons, when he hovered around 40 percent.
Of course, exit velocity matters most when it's combined with launch angle, because hitting the ball really hard into the ground is probably still going to be an out. So we've combined the two to identify six types of batted-ball contact, three of which are good for the pitcher (weakly hit, popups or topped into the ground), and three of which are good for the hitter, including barrels. The three "unproductive-contact" types have a Major League average of .139 this year, while the the three "productive-contact" types have result in an average of .652.
As you'd expect, there's an extremely strong relationship between the two, as most hard-hit balls are also productive balls. But for Bradley, even though he's had 49.3 percent of his batted balls qualify as "hard hit," only 32 percent of his batted balls have been "productive," and the gap between those two numbers is the largest of any player in the bigs right now.

Biggest gap between hard-hit and productive batted balls in 2017
17.3 percent -- Bradley, Red Sox
13.7 percent -- Joc Pederson, Dodgers
12.5 percent -- Yandy Diaz, Indians
12.4 percent -- Joey Gallo, Rangers
11.4 percent -- Manny Machado, Orioles
10.5 percent -- Greg Bird, Yankees
10.5 percent -- Cody Bellinger, Dodgers
9.9 percent -- Trea Turner, Nationals
Why? This isn't a Ryan Zimmerman situation, where the Nationals slugger was merely pounding all of his hard-hit balls into the ground; Bradley has actually lowered his groundball rate this year. It's not about making contact, either. Bradley's 23 percent strikeout rate is comparable to last year's mark, and it is below his career average. He's chasing fewer pitches outside the zone, and he's making contact with way fewer, down from 62 percent to 41 percent, one of the biggest drops in baseball.
That's important. One of our newest Statcast™ tools is the ability to take quality of contact (the combination of exit velocity and launch angle) to see how likely a given batted ball would have been a hit, independent of what the defense does, and roll in real-world strikeouts and walks to get a truer view of a batter's performance.
We do that because when a hitter like Bradley crushes a ball 412 feet for an out, as he did in Detroit's massive center field on April 8, we want to give him credit for the skill that goes into squaring up a ball that turns into a hit 92 percent of the time… even if JaCoby Jones had plenty of room to haul it in.

So what we can do is compare a hitter's Expected wOBA to their actual in order to see if they're overperforming or underperforming. ("wOBA" is Weighted On-base Average, which is very similar to traditional OBP except it gives more weight to extra-base hits than walks and singles, instead of giving equal credit to all times on base. The 2017 Major League average wOBA is .322.)
Last year, Bradley's expected wOBA was .336, which is above average, and his actual wOBA was .360, which is very good. Last year, for example, that topped Bryce Harper, Evan Longoria and Carlos Correa. This year, his expected wOBA is .315, which is slightly below average, and his actual wOBA is a miserable .281. Of the 228 hitters with as many plate appearances as Bradley, that's the 23rd-largest negative gap. If he slightly overperformed last year, he's very much underperforming this year.
Now, all this is saying is that he's "deserved" more than he's earned. It doesn't guarantee that more success is coming. But it's worth noting that after Bradley returned from a knee injury in April, still wearing a knee brace for weeks after, things went quickly downhill from there. Between his April 21 return and when his rolling exit velocity really bottomed out on May 4, he hit merely .154/.233/.231. Since May 5, Bradley has been showing increasing exit velocity and, more importantly, power, hitting .213/.288/.447.

It's still not a guarantee of success, and we saw Bradley struggle down the stretch last year after a red-hot first half. He'll always be known more for his outstanding defense than his bat. But it's too soon to give up on Bradley. After all, it's difficult to hit the ball as hard as he does as often as he does without adding strikeouts and not find success. The corner may have already been turned; we just haven't seen it yet.