The veteran utilityman was in the process of coming back from a strained left groin when the Yankees traded him to Boston for Stephen Drew.
"I'm glad to be off the DL," said Johnson. "It's always brutal to be on the DL. A lot has happened on this trip to the DL that I didn't really see coming. I'm happy to be able to be here and get on the field and be a part of the team."
To make room for Johnson on the roster, the Red Sox sent top prospect Mookie Betts back to Triple-A Pawtucket. Betts had only started two games since rejoining Boston on Aug. 1.
"He's a developing player, and in the two stints he's had here with us, you can see the improvements defensively but we didn't want to stall that development any further," said manager John Farrell.
How will the Red Sox use Johnson?
"Pinch-hit, able to play first, third, left field, so there's versatility there," said Farrell. "Kelly's aware of the role he comes in, so we'll pick those spots accordingly."
Johnson played for Farrell with the Blue Jays from 2011-12, and that staff included current Red Sox coaches Torey Lovullo and Brian Butterfield.
"That makes you feel a little more comfortable and they kind of know what you're about, but at the same time, I'm always looking to improve and try to be better," said Johnson.
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.