How good can Joe Kelly be in a full-season out of the bullpen?

May 21, 2016; Boston, MA, USA; Boston Red Sox starting pitcher Joe Kelly (56) reacts after pitching during the fifth inning against the Cleveland Indians at Fenway Park. Mandatory Credit: Bob DeChiara-USA TODAY Sports
May 21, 2016; Boston, MA, USA; Boston Red Sox starting pitcher Joe Kelly (56) reacts after pitching during the fifth inning against the Cleveland Indians at Fenway Park. Mandatory Credit: Bob DeChiara-USA TODAY Sports /

After transitioning to the bullpen, Joe Kelly dominated during the second-half and the 2016 ALDS.

Until August, the Boston Red Sox trading John Lackey to the Cardinals for Joe Kelly and Allen Craig has been an unmitigated disaster. Make no mistake, it still is right now. But Joe Kelly’s development as a relief pitcher provides the opportunity to reclaim some of that lost value.

By now, we’re all too familiar with Kelly’s story since coming to the Red Sox. Acquired on July 31, 2014, he went 4-2 with a 4.11 ERA in 10 starts. The front office and John Farrell continually insisted that he was a starter, despite his performance showing otherwise. In 2015, his now infamous self-prophesied Cy Young season saw him start on the disabled list, then pitch to a 5.94 ERA in his first 16 starts. There has been speculation over the course of his career, and certainly his first two years in Boston, that Kelly is better suited for the bullpen than the rotation.

The team finally gave in to the speculation in 2016, sending him down to the minor leagues on multiple occasions to grow accustomed to his new role. As of July 25th, Joe Kelly officially became a major league reliever – a role he appears to be perfectly suited for. In 14 games out of the ‘pen for the Red Sox, Kelly had a 1.02 ERA, 1.019 WHIP, and held opponents to a .558 OPS. As a starter, he owned an 8.46 ERA, 2.239 WHIP, and a 1.001 OPS against.

Obviously, 14 games is a small sample size, but Kelly maintained his level of performance in the ALDS as well. He retired all 11 batters he faced, giving a newly developed slider credit for his dominance in October. That could be a key pitch for him moving into 2017, as he induced swinging misses over 40% of the time he threw it. The addition of a power slider (it sits at 91 mph) adds to his high-90s fastball, mid-90s sinker, and slow curve.

Regular season and playoffs combined, Kelly has faced just 80 batters as a reliever. It’s hard to extrapolate what 2017 may look like for him as a reliever given the small body of work, but he’s given every indication that he deserves a late-inning role in Boston’s bullpen. As a starter, Kelly was always a tweak or an adjustment away from finding success. Because of his big fastball and decent secondary pitches, teams were willing to give him a shot at the rotation – even though he had never posted an FIP or xFIP below 4.00 in that role.

The easy explanation for starters-turned-relievers finding success is an increase in velocity. Less pitches = more effort = increased velo. Makes sense, right?

That’s partly true for Kelly, he averaged 96.7 mph on his fastball as a starter and sat roughly 98 mph as a reliever. But in his case, velocity had never been the problem – in fact, it was what kept him in the rotation for so long. He was already approaching his max velocity as a starter, putting him in the pen only added an extra mile per hour or so on average.

Kelly’s large-scale improvement is likely a product of only facing the same guy in the lineup once. As a starter, Kelly’s xFIP was 3.23 the first time through the order, in subsequent at-bats his xFIP balloons to 4.20. The peripherals tend to back up this claim as well. As a starter, Kelly walked 15.9% of batters he faced. As a reliever, just 7.24%. His strikeout rate increased from 22.6% to 30.4% as a reliever as well. Only 22 qualified relievers had a strikeout rate better than 30% last season.

It’s safe to say that Kelly belongs in the ‘pen, but can he maintain that success for a full season?

The good news moving forward is that there is historical evidence to believe that he can succeed as a reliever. Jose Contreras, Wade Davis, even Andrew Miller all posted numbers similar to Kelly’s as starters before successfully converting the bullpen. The reality is that it’s just easier to pitch out of the ‘pen than it is to be a major league starter. Nevertheless, expectations should be tempered.

Anticipating him to put up the type of numbers he did in such a small sample size last season would be foolish. If he did so, Kelly would be one of a handful of the best relief pitchers in the game. But I’d be lying if I said I don’t trust him to at least be a reliable middle inning guy. That’s a remarkable improvement over the sense of anxiety I would get seeing his name on a lineup card every five days.

With the addition of Tyler Thornburg and the incumbent Craig Kimbrel, the back of Boston’s bullpen has already been solidified. But based on what we’ve seen out of Joe Kelly in the second half and postseason, he might have the tools as a reliever to compete for some of those innings later in the game.

Next: Top-5 center fielders in Red Sox history

The Red Sox are quickly developing one of the sneaky best bullpens in the American League, in addition to baseball’s most productive lineup from a year ago, and a three-headed monster at the top of their rotation. The start of the 2017 season can’t come fast enough.