Rosario has great life on his fastball from the left side. It jumps out of his hand and can touch 96 mph. On the other side of that, Rosario struggles to control it, walking more batters than he struck out.
Few pitches can make Ronald Acuña look like he’s swinging a bat with a hole in it. Rosario’s slider struck out 35% of opposing hitters, with a 40% whiff rate.
Rosario’s four-seam fastball sunk enough to generate ground balls on half of his batted balls against the pitch last season. Yet there’s ultimately not enough movement to flummox big league hitters, who clubbed it for four homers and a .407 wOBA. It’s also not an option he can consistently control (54.3 zone %) or turn to for a punchout (6.9% strikeout rate).
Despite an unseemly 4.0 K-BB% in his rookie campaign, Rosario salvaged a 3.66 ERA with help from a slider that sliced out a .205 wOBA and 14.2% swinging-strike rate. He used it more than a lackluster four-seamer in August and September, but yielding 12 runs in 14.2 frames might discourage the 24-year-old from continuing that experiment.
An introduced sinker submitted a 64.5% ground-ball rate and a higher swinging-strike rate (8.7%) than his four-seamer (5.5%). He also ceded a .333 average (11-for-33), leading to a 0.2 pVal. The 24-year-old southpaw may need those grounders to survive his subpar 15.0% strikeout rate last season.
Rosario began to tinker with a changeup that surrendered no extra-base hits in just 31 throws. Eliciting six whiffs in the highly limited sample, it could be something the former starter tests out more in the bullpen.