His presence gives the Red Sox an odd bullpen dynamic in which they have four lefties and three righties.
Thornton and Craig Breslow can be used more on a situational basis, while Franklin Morales and Drake Britton offer length.
"It'll be a first to have that number of guys with still just a seven-man 'pen. If it was an eight-man 'pen, it'd be different. But we feel like we have length with Franklin," said manager John Farrell. "Just the fact that we get Thornton back active to us, that's the biggest bonus in this right now. He threw the ball well in the [simulated] game the other day. The ball is getting out of his hand good. Yesterday, there was no lingering effects or soreness. He's ready to go."
To make room for Thornton on the roster, the Red Sox optioned right-hander Brayan Villarreal back to Triple-A Pawtucket.
After being traded to the Red Sox just before the All-Star break, the last thing Thornton wanted was to have to miss time due to an injury.
"I've been a glorified cheerleader on the bench," said Thornton. "It's hard for me to be on the disabled list. I don't enjoy it. I was disappointed when it happened, but I'm very excited now that I'm going to be available tonight and hopefully available for the rest of the season."
Thornton no longer feels any effects of the injury.
"Great, I feel good," Thornton said. "I threw a simulated game on Friday and had no limitations, no problems afterward, no problem the next day. That was the big test -- amping up the intensity and repetitions and then how I felt the next day. I cleared both of those hurdles."
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. This story was not subject to the approval of Major League Baseball or its clubs.