We had a pretty good idea of who Trevor Cahill was: owner of a career 4.13/4.27 ERA/FIP, back-end starter, ground ball pitcher. He lost his rotation spot while pitching for the Diamondbacks in 2014. During 2015, he was released by the Braves after a failed transition to the bullpen, and, after opting-out of a minor league contract with the Dodgers, there was the possibility that this might be the end of any meaningful career for him. Still only 27 years old, he wouldn’t have been the first fringy starter to flame out of the league.

In late August, however, he signed a minor league deal with the Cubs, and two weeks later they called him up to the major league bullpen. Something pretty drastic had happened over the course of his time in the bullpens of the three teams he was employed by during 2015, and it all seemed to culminate in his 17 September and October innings for Chicago: he posted a stellar 27% K-BB% during that stretch, returning successfully to his ground ball ways (61.8%) and a 2.12/3.13 ERA/FIP.

Those 17 innings were, of course, a tiny sample size. But in those innings, as well as his successful work during the playoffs, we glimpsed who the new Cahill might be, and it was the pitcher the Cubs think they just signed to a low-risk, one-year, $4.25 million deal last week.

First, we saw a big velocity jump from Cahill in 2015. He almost exclusively throws a sinker as his main fastball, and he increased its velocity in 2015 by about two and a half mph from its highest point in 2014. Take a velocity look at a chart for his sinker for the months of 2014 and 2015, courtesy of Brooks Baseball:

His other pitches saw the same sort of gains, jumping around two mph from their previous highs in 2014. The usage rates of his pitches also changed: he junked his cutter/slider completely (it was labeled a cutter by PITCHf/x, but other sources refer to it as a slider) when he moved to the Cubs, going to a repertoire of only a sinker, curveball, and changeup. Check out the change in his pitch usage rates for 2014 compared to his stints with the Braves and Cubs in 2015:

Trevor Cahill Pitch Usage, 2014-2015 Season/Team Sinker% Fourseam% Cutter% Curve% Changeup% 2014 — Total 58.1% 5.0% 2.6% 15.2% 18.9% 2015 — Braves 52.9% 2.7% 22.0% 10.8% 10.8% 2015 — Cubs 53.3% 4.3% 2.2% 16.0% 24.2% SOURCE: FanGraphs

More velocity, dropping a pitch that doesn’t work and simplifying the repertoire; these are the types of changes converted starters often make when moving to the bullpen. With the Cubs, he basically went back to what he was doing in 2014, only with a few more changeups in place of sinkers. Cahill was working on an overhaul of his mechanics in the spring of 2015 as well, and that adds another interesting wrinkle to his 2015 campaign. Eno went over it in more detail last April, but the basic gist is that Cahill moved to a more over-the-top arm slot. Sahadev Sharma at Baseball Prospectus spoke with Cubs’ catcher Miguel Montero during the playoffs about some of Cahill’s changes:

“…as soon as he [Cahill] came here, I sat down with him and I said, you know what, I heard you were changing your delivery in Arizona in Spring Training, I heard they changed your arm angle and whatnot, blah blah blah. I want you to be you. Just go out there and throw the ball. Just be you and don’t worry about the rest. I mean, he’s been impressive, man. He’s probably been as good as I’ve ever seen him.”

While the right-hander did go back to his old pitch repertoire (Montero caught Cahill while the two were in Arizona), the catcher didn’t convince him to ditch the arm slot changes, as the new mechanics stuck for the entirety of 2015. Cahill released the ball on average about four inches higher for his sinker than he did in 2014, while his other pitches showed slightly less modest gains in horizontal release point:

We usually see movement differences with arm slot changes, but interestingly, they didn’t show up in the way we might expect them to. Cahill actually got less vertical movement on his sinker and changeup with the new slot, to the tune of an inch and half an inch, respectively. However, he did get a little more run (horizontal movement) on his pitches, especially on the changeup, which had almost an entire inch of greater fade. Less sink on the sinker might actually not have been a big deal considering the large increase in velocity, which has been shown to correlate with an increase in ground ball rate. That would make sense, given the fact that last season he posted the best ground ball rate of his career since 2012.

How did all of these changes translate to whiff rates and effectiveness of individual pitches? In interesting ways. Next, let’s look at his whiff rates and ground ball rates for 2014 vs. his two stints in the majors during 2015:

Trevor Cahill Pitch Peripherals, 2014-2015 Season/Team Sinker Whiff% Sinker GB% Changeup Whiff% Changeup GB% Curve Whiff% Curve GB% 2014 — Total 5.4% 51.9% 22.6% 47.7% 17.7% 71.4% 2015 — Braves 2.5% 65.4% 14.9% 50.0% 12.5% 50.0% 2015 — Cubs 4.1% 63.0% 37.1% 66.6% 18.9% 0.0% SOURCE: FanGraphs

The 2015 stints, again, are small sample sizes. We know this. But they do mean something, and the effectiveness of the changeup especially draws our eye in light of the changes we’ve seen above. Cahill was the victim of both bad and good “fortune” in 2015 — his BABIP for the Braves was .354 while it was .182 for the Cubs — but he got a lot more whiffs on his main pitches, showed better command of the strike zone, and induced more soft contact (19.4% vs. 31.4%, respectively) with the Cubs. That was enough for the team to offer him a contract.

Maybe Cahill’s last two months of 2015 was a mirage, and he’ll revert to something closer to his less effective ways. But given how many changes he’s made, the increase in velocity, and the results he’s shown in that small sample, we should at least be intrigued enough to watch his 2016 campaign with interest. There could be a very good bargain here for the Cubs — a capable, ground-balling middle reliever who can possibly step into a spot start or two should the need arise. That’s not a bad guy to have around, especially given the price. Much stranger things have happened than a formerly ineffective starter becoming a good reliever.

For Cahill, maybe this is another evolutionary step in a game that requires a lot of them. “It’s been a crazy year, a long year” he told FanGraph’s David Laurilla last month. Perhaps, with these changes under his belt and a little luck, next year will be a little less crazy, and a little longer still.