Although he has not thrown an MLB pitch in 2018, Sean Doolittle threw 4,610 pitches that were tracked by the PITCHf/x system between 2012 and 2017, including pitches thrown in the MLB Regular Season, the MLB Postseason and Spring Training. In 2017, he relied primarily on his Fourseam Fastball (95mph), also mixing in a Slider (82mph). He also rarely threw a Change (86mph) and Splitter (86mph).
Basic description of 2017 pitches compared to other LHP:
His fourseam fastball generates an extremely high number of swings & misses compared to other pitchers' fourseamers, is blazing fast, has good "rising" action, has less armside movement than typical and results in somewhat more flyballs compared to other pitchers' fourseamers. His slider generates more whiffs/swing compared to other pitchers' sliders, results in more flyballs compared to other pitchers' sliders and has short glove-side cut. His change is an extreme flyball pitch compared to other pitchers' changeups, is much firmer than usual and has an extreme amount of backspin. His splitter has movement that suggests an extreme amount of backspin, is slightly firmer than usual and has slight armside fade.