Many players have had breakthrough seasons in 2015, but these are the nine breakouts I believe are most sustainable into future seasons. I picked four starting pitchers and five position players.
Chris Archer, SP
Tampa Bay Rays
2015 stats: 2.77 ERA, 169 IP, 42 BB, 205 K's, 4.0 WAR
I mentioned Archer in my 2014 predicted breakouts column, saying he should jump into the top 10 in strikeout rate; I guess I was just a year off, as he's third in the majors in strikeouts as a percentage of batters faced, behind only lefties Chris Sale and Clayton Kershaw. His slider, now often as hard as 91 mph, has become one of the majors' best, although a big part of its effectiveness is his improved fastball command, so he's getting ahead in the count more frequently and staying there as well. For one example, he has gone to 65 2-0 counts so far this year, after doing so 110 times last year. Getting ahead with your fastball so you can finish hitters off with your off-speed stuff is an old formula for pitching success that I doubt will ever change -- because it works -- and it has helped Archer become a top-five pitcher in the American League.
Shelby Miller, SP
Atlanta Braves
2015 stats: 2.50 ERA, 158 1/3 IP, 56 BB, 136 K's, 4.1 WAR
One of my breakout candidates for this season, Miller could stop pitching now and it would already be a career year for him, as he's doing all three of the things essential for an ace: missing bats, avoiding walks, and -- for the first time since he reached the majors -- keeping the ball in the park by making hitters put it on the ground. Miller's ground-ball rate jumped from about 39 percent over the past two years to 50 percent this year, thanks to the switch to a two-seamer along with a cutter, while his curveball has become more effective as he has had to use it less frequently. If he's not quite an ace, he's quite close to it, and Atlanta has him under team control for the next four seasons.
Carlos Martinez, SP
St. Louis Cardinals
2015 stats: 2.85 ERA, 148 2/3 IP, 52 BB, 150 K's, 3.1 WAR
Martinez was effective in a relief role for the 2013 and '14 Cardinals, making just eight starts over about a year and a half of major league time in that span, and there were reasonable questions about whether he could start or would get that opportunity. He had a 4.86 ERA in those eight starts with a walk every other inning, although I doubt the Cards read that much into so small a sample. Martinez won the fifth starter's spot in spring training and has become a huge part of their rotation in the wake of Adam Wainwright’s season-ending injury at the end of April, perhaps second only to Michael Wacha in performance this year. Everything Martinez is doing seems sustainable: He has three above-average pitches, he misses bats, he throws strikes, he keeps the ball down. The only question I might have is his durability, as he's a smaller guy with some arm woes in his distant past. I like how his arm works, though, and I think he's a top-of-the-rotation guy going forward.
Nathan Eovaldi, SP
New York Yankees
2015 stats: 4.00 ERA, 144 IP, 43 BB, 107 K's, 2.6 WAR
Eovaldi's breakthrough doesn't go all the way back to Opening Day, but he's visibly different now than he was when the Yankees first acquired him. How? He now has a third pitch -- the splitter -- that he lacked when he was with the Dodgers and Marlins. Eovaldi was always blessed with a huge arm, but the lack of deception in his delivery and absence of a change or split meant lefties killed him, and even right-handers made more contact than they should have. Enter the splitter, which has been the most effective in baseball per FanGraphs' pitch values (although they misclassify it as a changeup; Pitch F/X correctly calls it a split). Eovaldi has thrown 350 of them, and hitters have swung and missed at 62, more than twice the rate at which hitters are putting it in play for hits. The 13-2 won-lost record means nothing, but the improvement he has shown over the past two-plus months, missing more bats and letting fewer hitters square up his fastball, is real.
Bryce Harper, OF
Washington Nationals
2015 stats: .330/.455/.640, 31 HR, 7.7 WAR
Harper is having a historically great season for a player his age; Baseball-Reference has Harper's 2015 as the 22nd best season ever by a player aged 22 or younger as measured by WAR, while his slugging percentage (third) and OBP (fourth) both rank among the top five ever by a player in that age range, behind only players who ended up in the Hall of Fame. We've been privileged to watch Harper -- still called "overrated" in a player poll this spring, more evidence that all player polls should be fired into the heart of the sun -- as he matured as a player over the past three years, improving his approach at the plate by leaps and bounds. He has become one of the best hitters in the majors against sliders in addition to still murdering fastballs. Opposing teams used to have some avenues to get Harper out, but he has closed them off, hitting lefties, hitting soft away, and spitting on pitches just out of the zone. The Nats' collapse might hurt his MVP chances, but there's no question in my mind that if the season ended today he'd deserve the award in the NL.
Gregory Polanco, OF
Pittsburgh Pirates
2015 stats: .267/.334/.399, 7 HR, 2.0 WAR
I was getting tweets and chat questions from readers in the first half asking what was wrong with Polanco or even if he was a bust, but I think that's a symptom of how the immediate successes of players like Harper and Carlos Correa have spoiled us. Many highly rated prospects take more time to perform up to the hype under which we've all buried them, and Polanco appears to be another example. He has been a fairly high-contact hitter throughout his career, even when he wasn't seeing results in the majors, but he's making even more contact now and being more aggressive early in the count. He can crush a fastball, and has done so more effectively of late because he's looking for that pitch, rather than letting himself fall behind by being too passive. I'm a longtime believer in his tools, and still think there's power to come, but the first step was improving that part of his approach.
Xander Bogaerts, SS
Boston Red Sox
2015 stats: .313/.342/.408, 4 HR, 3.4 WAR
He hasn't been the player I expected him to be, at least not yet, but Bogaerts has started to show progress at the plate by becoming more aggressive -- the way Polanco has -- also seeing fewer pitches per plate appearance but in the process putting far more balls in play because he's attacking pitches in the strike zone (up from 60.5 to 65.6 percent per FanGraphs). Bogaerts has also benefited from getting back to his natural position of shortstop, where he has played solid-average defense this year and at least doesn't have to work on learning a new position while he's adjusting to major league pitching. That said, the current approach can't be permanent for Bogaerts, who has walked just 3.9 percent of his PAs this season; he needs to find his middle ground where he can be patient without becoming passive. I still think he's got All-Star upside, but to get there he'll have to walk more and also start driving the ball to the gaps more.
Joe Panik, 2B
San Francisco Giants
2015 stats: .309/.374/.443, 7 HR, 3.2 WAR
For all the adulation, mostly deserved, that Jose Altuve gets, Panik's 2015 season is a better version of what Altuve has turned in, just with more walks. I was never a Panik fan at all, as he was a no-power slap guy who didn't walk and couldn't play shortstop well enough to be an everyday guy there, but played it just well enough to be a utility infielder. (I don't think you can be a utility infielder in the era of the 12-man pitching staff unless you can handle short. It's a lower rating than calling someone a regular, but it's a higher standard than it was 15 years ago.) But Panik has become almost impossible to strike out -- he's one of only eight qualifying big leaguers to strike out in less than 10 percent of his plate appearances, and with a career BABIP of .336 now in over a full season's worth of at-bats, he seems likely to continue to hit .300-plus because of his contact rates. Even if he gives back some of the power, which seems likely given his swing and entire pro career to date, he's still likely to be an above-average regular at second base for the next six to eight years.
Brandon Crawford, SS
San Francisco Giants
2015 stats: .267/.326/.485, 19 HR, 5.8 WAR
I would have considered Crawford more strongly a few weeks ago, but now I'm not so sure if this is sustainable. The only real difference between this year and his previous few is his home-run output; he's actually walking a little less often as he's no longer hitting eighth. His defensive performance does rate higher this year, but in fairness to Crawford, he has always been an outstanding defender at short. So it's somewhat sustainable, at least for the defensive part, but I would expect some regression in his power output and an OBP closer to .300 next year.