"On the second pitch [of the outing], I felt a little cramp," Thornton said on Wednesday. "The next one hurt a little bit and the fourth one took my breath away. Had a good idea then I was hurt."
The Red Sox acquired Thornton from the White Sox on July 12, and the 10-year veteran has made 10 appearances for his new club, giving up two runs on 12 hits over 8 1/3 innings with two walks and six strikeouts. In 50 total appearances this year, he has a 3.47 ERA, holding left-handed hitters to a .221 average.
Thornton and manager John Farrell said the southpaw had improved each day, but the team didn't want to aggravate the injury.
"Really, when you get into an oblique injury, even though he feels improved from Sunday, this is something we don't want to rush with the potential for a setback," Farrell said. "The fact is, we also need an arm in here after last night's bullpen use. To project a return to game activity will be around two weeks even with the improvement he's shown."
Thornton tossed the ball around the outfield before Tuesday's 15-10 win against the Astros, but he backed off Wednesday with more time to rebuild his arm strength. The veteran said he's not trying to overextend himself with the Red Sox in need of his services come September and beyond.
"Talking to the guys that have had it -- I've never had anything like this in my career before -- and everyone said they tried to come back too quick and it doubles the time coming back again," Thornton said. "So I didn't want that. I need to be fresh and really healthy when I'm back out there."
Thornton said extensive cardio and stretching regimens with the training staff will be his primary rehab methods, saying consistent blood flow to the region should make the injury a "minor thing."
This is the fourth big league stint of the season for Beato, who made nine appearances and allowed three earned runs over 8 2/3 innings. The 26-year-old has a 2.09 ERA in 26 relief appearances for Pawtucket.
Chris Abshire is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.