Beckett was on the DL from July 8 to July 22 with a left hip impingement and is 0-1 with an 8.25 ERA in the three starts since his return. A previous MRI revealed a torn labrum and two cysts in the hip. The exact nature of the current injury has not been disclosed.
"I guess they'll wait for the MRI," Mattingly said. "I'm assuming it's got something to do with his hip in some relation. But it's a different area that was hurting him."
Right-hander Roberto Hernandez, acquired Thursday from the Phillies, will start Friday in Milwaukee in Beckett's stead. General manager Ned Colletti said he had been looking at Hernandez for a couple of weeks, but talks with Phillies general manager Ruben Amaro heightened after Beckett's injury.
Rookie right-hander Carlos Frias would have gotten the ball Friday if the Dodgers had not traded for Hernandez.
"Well, Josh has been battling some things lately, and it came to the point where we had the chance to get Roberto," Colletti said. "And this time of year, as I've said many times, you better have enough of everything to get you through it."
Michael Lananna is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.