Daniel Nava was activated to help fill the void for left fielder Carl Crawford, who will undergo season-ending Tommy John surgery on Thursday.
And righty reliever Vicente Padilla was activated the first day he was eligible after a bout with right arm and groin tightness.
Nava was immediately installed in manager Bobby Valentine's lineup, batting seventh and serving as the DH. He had been out since July 29 with a left wrist sprain.
In 65 games this season, Nava is hitting .251 with four homers, 27 RBIs and a .373 on-base percentage. Earlier in the year, he was one of Boston's best hitters. His numbers started to decline around the time his wrist started aching.
"Regardless of whether [Crawford] was going to have this [surgery] or not, getting activated and participating and being able to contribute in some way, shape or form was what I was trying to do," said Nava. "I'm not trying to repeat what I was doing earlier. I'm just trying to go out and help the team win tonight or tomorrow."
The Red Sox will miss Crawford, who was a far better player in his short stint this season than he was in 2011.
"We'll miss Carl," said Valentine. "I'm really happy that Carl got the games in that he got in, and I'm really happy that he had the success that he had. I think Carl now can feel that at least he showed the fans in a small way, that in an everyday way, that he's a very good player. I think he needed to do that."
Padilla gives Valentine another right arm he can depend on in the late innings.
"Supposedly, he's over the hump with the little groin situation that he had, which kind of caused a little triceps-biceps strain," Valentine said. "They fixed the bottom, and I think they fixed the top."
To make room for Padilla on the roster, the Red Sox optioned third baseman Danny Valencia to Triple-A Pawtucket.
"Danny was in about as unfair a situation as I've seen anyone. He played a couple of games in 10 days," said Valentine. "We tried to throw him in the fire, and then he went to the bench. I'm hoping he can get some consistent at-bats in Triple-A and still help us this year."