"After that, it started getting worse and worse," Thornton said. "It got more painful instead of a cramp."
Manager John Farrell and a trainer came to the mound and decided to take him out of the game.
"We'll check him tomorrow to see the severity of it," said Farrell, "but he felt it in the second pitch of that final at-bat and then a couple of pitches after that. You could see from the dugout he was starting to get a little bit more upright and protected. We clearly had to get him out of there."
Thornton, who has appeared in 10 games and has 2.16 ERA since joining the Red Sox in early July, said he felt good after the game. He met with team doctors and was told that how he feels Monday will be a determining factor on if he'll need to miss any time.
"They said that tomorrow is the today we have to worry about," he said. "We'll see how it feels tomorrow and figure it out."
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. Michael Periatt is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.