His fastball has never had great shape or movement, but he threw 95, so it didnt matter as much.
If he's going to be even remotely effective, he's going to have to re-develope his fastball.
I don't know what Brooks is showing, but on FG, it looks like he was throwing 2 fastballs, a 4 seam and a sinker, as well as a curve and a change. All about 25% of the time. The sinker is the newest to be emphasized. From 2013 -2017 he didn't throw it more than 3.5% of the time, which may just be misclassification. 2019 is when the sinker and curve % shot up to nearly half his pitches.
I'll guess he's seeing some of the same things Slate is seeing. With the velocity drop, his 4 seam can more or less be a variety / different look, while the sinker is the fastball with some movement. identical velocity on the fastballs. Sinker wasn't working his first start, but was more effective than his old fastball in 2019 (and the short sample in 2021).
https://www.fangraphs.com/players/stephen-strasburg/10131/stats?position=P
