Nava, who is off to a strong start at Triple-A Pawtucket, is a switch-hitter.
"Our outfielders have really been put to task, playing as much as they have been, and another left-handed bat to spell our right-handers, I thought, was necessary," said Red Sox manager Bobby Valentine. "Daniel's been real hot. [We] got great reports on him from Arnie [Beyeler, Pawtucket manager] and everybody who's seen him play, seemed like the right choice."
Nava, who wasn't invited to Major League camp during Spring Training, fell off the radar a bit. Now he has a chance to reemerge.
"I'm obviously happy to be back," Nava said. "I learned a lot last year, struggling and trying to find myself as a hitter again, just a lot of things, putting baseball in perspective. I'm glad to be back."
To make room for Nava on the 25-man roster, the Red Sox had to option Clayton Mortensen back to Pawtucket. Valentine informed him of the move in the wee hours of Thursday morning, after the Red Sox returned from Kansas City.
"About 4:45 this morning, when I called him into my office, I told him baseball and life isn't fair," Valentine said. "And it's not -- nowhere does it say it has to be fair, and this isn't fair to him. He pitched plenty well enough to be on this staff, but he has an option and we needed a spot, and luckily he's the kind of professional guy that understood it totally."
The Red Sox also had to add Nava to the 40-man roster, and to do so, they designated lefty Justin Thomas for assignment.