The 24-year-old right-hander (0-1, 7.71 ERA) left the mound in the bottom of the first inning after appearing to suffer the injury during warm-ups.
Sampson will be evaluated back in Seattle.
"Was no pop or anything, just a little discomfort. Felt it was best if I just come out of the game,'' Sampson said.
He was replaced by reliever Vidal Nuno, who has a 1.95 ERA in 26 games for the Mariners this season. Nuno last made an appearance in Monday's series opener against the Tigers, during which he gave up one run on two hits in two innings of relief.
Nuno started for the Mariners and pitched into the fourth. Seattle used six pitchers overall in the 5-4 loss in 10 innings.
The Associated Press contributed to this report.