Bay returned to Boston's lineup Wednesday at Tampa Bay after missing two games with a mildly strained hamstring. But he left the game after aggravating the injury running out a grounder in the eighth inning.
Bay told reporters after Wednesday's 6-4 loss to the Rays that he "has no idea" how much time he'll miss.
"It doesn't feel real bad. Anytime you can't play, you feel like you let your teammates down," he said, adding, "but it's definitely going to be a couple days. Basically the same thing. It had calmed down, and it kicked it right back to where it was three or four days ago."
Bay, who is hitting .252 with a team-leading 21 home runs and 75 RBIs, said he would be re-evaluated earlier in the day Thursday.
Information from The Associated Press was used in this report.