The Dodgers also activated outfielder Juan Rivera, purchased the contract of reliever Shawn Tolleson from Triple-A Albuquerque, optioned Scott Van Slyke to Albuquerque and moved Matt Guerrier to the 60-day disabled list.
Guerra was hit in the face by a line drive off the bat of Atlanta catcher Brian McCann on April 28 and twisted his knee, which has proven to be more troublesome over the past 1 1/2 months.
"He'd been having the knee stuff ever since he got hit," said Dodgers manager Don Mattingly, who had yet to see the MRI report before Monday's game against the Phillies. "It wasn't as much of a concern, and we thought he could get through all that until he started saying he tried to change his mechanics a little bit to compensate and his arm was getting a little sore.
"At that point, that wasn't going to work for us. The fact he was changing mechanics was kind of dangerous. This kid has too good of an arm to be messing with that."
Ken Gurnick is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.