Lester left Friday night's game with a possible leg injury after appearing to slip on the mound in the eighth inning.
Lester had settled down after allowing three earned runs against the Royals, shutting down Kansas City as the Red Sox took a 4-3 lead.
But when Lester took the mound in the top of the eighth, he left after throwing three balls to Royals designated hitter Billy Butler. He appeared to slip on the mound and was limping when he left the game.
Lester was replaced by Vicente Padilla.
The Red Sox have not released any news about Lester's condition.