The changes began immediately. In his first start as a Cub, Arrieta leaned far more heavily on his sinker than he ever had before, throwing 48 of them out of 93 pitches in his debut start against the Brewers. He walked three batters and struck out two, but he also allowed just one run on two hits that night, inducing lots of weak contact. Arrieta made eight more starts with the Cubs that year, producing four terrific outings (1.01 ERA) and four rough ones (8.05 ERA).
In 2014, Arrieta’s pitch mix continued to evolve. After using his four-seam fastball 35 percent of the time in 2011, he used it just over half that often three years later. He also started throwing his slider more, ramping it up to a career-high 29 percent usage rate. The pitch had elements of a cutter, and some have argued that it resembles more of a hybrid slider-cutter offering — inelegantly called a “slutter.” Whatever the name, that pitch became Arrieta’s needed third weapon. Take a look at opposing hitters’ heat map against it last year, and you see an ocean of icy blue — especially for lefties:
Of course, adding a successful third pitch can help any pitcher get a lot better. But that slider — and Arrieta’s other pitches — became so effective in the first place because of base-level changes to both his mechanics and process.
In 2014, Harry Pavlidis detailed Arrieta’s transformation for Baseball Prospectus. His pitches began hitting the strike zone more often — especially in parts of the zone that hitters couldn’t touch with any authority. So, why the improvement? Pavlidis shot a few videos of Arrieta throwing under the watchful eye of Cubs pitching coach Chris Bosio. You can see Bosio working with Arrieta on his balance, particularly as he breaks his hands apart and starts to shift forward in his delivery. And in an MLB Network Radio interview from the same year, Arrieta talked about making sure that his shoulders were level, he wasn’t leaning back toward second base in his windup, and his left leg was moving down before he started shifting his weight toward home plate — all elements he worked on with Bosio.