Mendez, 25, has struggled with control during his time in the Majors, issuing 24 free passes while also striking out 31 in 44 2/3 innings over parts of the past two seasons with the Rangers.
This year, he appeared in 12 games for Texas -- all in relief -- allowing seven walks and 11 hits while striking out nine in 11 2/3 innings and posting a 5.40 ERA.
Mendez was originally signed by the Red Sox out of the Dominican Republic in 2007 as an amateur free agent, and he was traded to Texas in a '10 non-waiver Trade Deadline deal that sent catcher Jarrod Saltalamacchia to Boston.
"We're happy to have him back, but we're not going to rush him into a game at this point," said Red Sox interim manager Torey Lovullo of Mendez, who hasn't seen any mound action in 10 days.
"He's coming in tonight, and you may see him throw a bullpen [session] just to get a little touch and feel. Knowing that he's had a little down time, we just want to steer him in the right direction and give him a chance to get his legs under him," Lovullo said. "This is a kid we're picking up, and obviously, we want to see him on the mound at some point."
Michael Kolligian is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.