"It did kind of flare up there early on. He did not have it coming into the game," Cubs manager David Ross said. "Those types of things, it's only a matter of time."
Stroman entered the game with an NL-best 2.28 ERA and was 7-0 in his past seven starts going into the series finale in London.
"You could just tell the pitches weren't as sharp as they had been, and we had a fully rested bullpen," Ross said.
Stroman allowed six runs, three earned, and eight hits -- an earlier hit was changed to an error -- and had two strikeouts and a walk.
"Knowing him, he'll be ready for his next start, but I have not talked to the trainers yet," Ross said.