Kershaw’s average fastball velocity Thursday was at its lowest ever, and it was his worst start by 1.4 mph. Kershaw’s previous low came in a 2012 start when he was pitching through the flu, so if we wipe that off the board. Thursday night was Kershaw’s worst start in terms of fastball velocity by 2.6 mph — a gap large enough to span Kershaw’s average fastball velocity from his previous worst non-flu game (91.7 mph) to his 232nd worst game (94.3 mph).
Kershaw’s velocity declined steadily through 2017, but Thursday’s start was beyond what anyone could have expected. And it wasn’t just back tightness; Kershaw said his back tightened up in the middle of his start, but his pitch velocity was down from the start.
You don’t need to have your velocity drop to be injured, and velocity can fluctuate from game to game. But drops of more than 2 mph should be cause for concern. Those kinds of drops don’t come from nowhere, and they’re often indicative of injury.