It's possible that Jackie Bradley Jr. is the fourth-most-notable player in his own outfield, given the presence of J.D. Martinez, reigning American League Most Valuable Player Mookie Betts and postseason heroAndrew Benintendi.
That's partially due to the sheer quality of the players who reside alongside him in baseball's best outfield, and it's partially due to the up-and-down nature of Bradley himself. He's had star-level seasons, hitting a strong .262/.345/.489 (119 wRC+) in 2015-16, with 36 home runs across the two years, and good defense; he's also had two straight disappointing years, hitting only .240/.319/.403 (89 wRC+) in 2017-18. Even his sparkling defensive reputation was somewhat up for discussion in 2018, as he won his first Gold Glove, but also had 0 Defensive Runs Saved.
Maybe we'll always be tantalized by Bradley's talent while also being frustrated that he's been unable to convert it into that season, the kind where he combines more offensive talent with elite defensive play. Maybe it will never happen... and maybe it already started to.
The last thing the Red Sox really need is another star-level outfielder. Here's how they may be in the midst of watching one rise, nearly six years after he first made his debut with the team. Bradley's underrated, at a minimum. In 2019, it might be a lot more than that.

A reason to finally believe in the bat
Bradley has nearly 2,500 plate appearances spread across six seasons. He's been around for so long that when he made his Major League debut in a Red Sox-Yankees game in 2013, Jacoby Ellsbury was still patrolling the outfield for Boston and Betts was a Class A second baseman. This isn't exactly a small sample size.
The results, to be blunt, haven't been encouraging. Bradley's career line is just .238/.317/.407, a 92 wRC+ that's eight percentage points below average. That's somewhat dragged down by the dreadful .196/.268/.280 (50 wRC+) he put up in 530 plate appearances in 2013-14, but as we noted above, his last two seasons weren't strong and lagged behind his 2015 and '16 anyway.
It's true that after a weak start -- .210/.297/.345 (71 wRC+) in the first half in 2018 -- Bradley was much better in the second half, hitting .269/.340/.487 (118 wRC+), a performance that was roughly as good as Nelson Cruz or Freddie Freeman after the break. But it's also true that we've seen the streaky Bradley do this before, a few times, and then balance it out with a crash right back to earth.

So what makes this time different? It's possible that it won't be, that when the 2019 season starts, it'll be more of the same. But if it feels like this could have really been the start of something, it's because there's more to point to than just outcome stats. There's a truly encouraging increase in hard-hit rate, to an elite level, and there's a pretty clear explanation that Bradley can point to as to what fueled his strong second half.
To start with, Bradley's hard-hit rate -- that is, the percentage of balls hit with an exit velocity of at least 95 mph -- skyrocketed in 2018, from 39.7 percent to 50.4 percent.

That's a big improvement, and an important one. There were 216 batters who had at least 200 batted balls in each of the last two seasons, and only five had larger jumps in hard-hit rate, including Betts.
If we set the 2018 minimum at 150 batted balls -- roughly one per team game -- then that 50.1 percent hard-hit rate isn't just good, it's elite, essentially tied with Betts and Shohei Ohtani for 10th of the 332 qualifiers. That's in the 97th percentile, and several of the names ahead of him are baseball's true sluggers, like Martinez, Aaron Judge, and Giancarlo Stanton.
Bradley, like every other hitter in the game, is far more successful the more often he hits the ball hard. In 2018, he hit .447 with an .871 slugging when hitting the ball 95 mph or harder, and just .204 with a .255 slugging when hitting it 94 mph or softer. This part isn't complicated, really: It's good to hit the ball hard as often as you can. For Bradley, his three hardest-hitting months since Statcast™ came online in 2015 all happened in the final four months of 2018.
If that had happened with no particular reason behind it, you might be tempted to toss it off as a mere hot streak. But in this case, it seems like Martinez may have offered a good deal more value than just the 43 homers he popped, because not only did Betts seek out Martinez's advice as he cruised to a career-best season, it seems that Bradley did the same.

Furthermore, Martinez introduced Bradley during the season to Craig Wallenbrock, the private hitting coach who helped turn around Martinez, Chris Taylor and others. This winter, he's spent part of his offseason working with Wallenbrock in person.
"This is the first time I heard any of this stuff," Bradley told WEEI.com in December. "What I've been taught my whole life is completely wrong. It's scary to say that, but it's wrong. I feel fortunate enough to make it this far doing it wrong.
"Knowing I can hit the ball just as hard as them physically, it all comes down to the way I impact the baseball. Well, I hit too many ground balls, so let's solve that problem. Let's get the ball off the ground, get it more in the air, on a line, and that way the shifts will be beaten."
If finding a hitter with above-average hard-hit skills and getting him to elevate sounds familiar, it should; it's been among the most prominent trends in baseball over the last few years. It doesn't always work, of course, but when it does, the results can be outstanding.
Daniel Murphy, for example, hit .290/.333/.419 over 2,855 plate appearances in his first six seasons, and .314/.361/.517 in the four years since. Justin Turner hit .260/.323/.361 in parts of five seasons until he was Bradley's age, and .305/.383/.505 since. That doesn't mean that Bradley will do the same, obviously. Then again, neither one of those guys played an elite center field.
The defense was already great
The DRS of zero -- i.e., "average" -- was somewhat surprising, because by the Statcast™ metrics, Bradley's defense remained elite. By putting up +11 Outs Above Average, he tied with Betts for the ninth-best mark in baseball.
For example, when Bradley robbed Adam Jones on April 14, it was a play that had a Catch Probability of under 10 percent.

In September, this diving catch to take a hit away from Caleb Joseph had a Catch Probability of just 13 percent, meaning that nearly 90 percent of the time, that opportunity isn't converted.

And in April, before the Red Sox had even made it north to Fenway Park, he took away a sure hit from Justin Bour on a chance that had a Catch Probability of only 36 percent.

It's worth noting that Outs Above Average doesn't currently include arm value, which means that Bradley might not actually be getting full credit for his skills. That's important, because on June 19, he unleashed a 103.4 mph throw to nail Robbie Grossman at the plate, the hardest tracked outfield throw of the year.

Put it all together, and ... 
OK, so what's the upside here? What if Bradley does hit the way his hard-hit rate increase and second half suggests he can, and plays strong center field defense, and continues the good baserunning skills that had him ranked as one of the 15 most valuable runners in 2018?
Well, we've sort of seen that already. In 2016, Bradley had what was to date a career year. He hit .267/.349/.486 (118 wRC+), almost exactly what his 2018 second half line was. His defense was rated just as well as you'd expect, with +11 DRS, +8.2 UZR, and +10 Outs Above Average.
That year, at FanGraphs, Bradley was worth +5.2 Wins Above Replacement. It made him one of the 20 most valuable players in the game that year. He couldn't maintain it, obviously. But we've seen him do it before, and now he's working with an improved skill set, one with a career-high hard-hit rate that's bordering on elite, and with a hitting coach with a proven track record of helping decent hitters become strong ones. We've been tempted by Bradley's skills before, and it hasn't always worked out. There's enough happening here to think this time might be different.