Scherzer had been cruising when, with one out in the sixth inning, he threw a pitch to Bo Bichette and then reacted on the mound. Mike Maddux, Bruce Bochy, and the trainer all went out to the mound, and after Scherzer started twice to throw a phantom pitch, and stopped, he left the game. Jose Leclerc came in in relief.
I initially thought it was a lower body issue, and speculated it was groin tightness, based on how he appeared initially. Watching a replay, however, you could see him say it was his triceps, and that is the official announcement from the Rangers as well.
Scherzer had allowed just three hits and one walk in his outing prior to being lifted. His spot in the rotation would next be on Sunday in Cleveland, though with Martin Perez, Andrew Heaney and Cody Bradford all in the bullpen, the Rangers could well opt to push him back and use a spot starter for that game.