The club had announced after Friday's game that Beckett would miss his scheduled start on Sunday night at Wrigley Field against the Cubs.
"He's got some inflammation that's got to calm down and take the time, whatever time it takes to do it," said Red Sox manager Bobby Valentine. "We thought in between starts was enough, and he could have gotten an extra day or two [of rest] I guess, but we're going to take the conservative route."
Right-hander Clayton Mortensen, a reliever, was recalled from Triple-A Pawtucket to take Beckett's spot on the roster. While Clay Buchholz could have pitched in Beckett's place on Sunday on regular rest, lefty Franklin Morales will come out of the bullpen to make that start.
"Of course, yeah, yeah, that was Plan 1, but Clay felt he needed the extra days," said Valentine. "Well, you know, he pitched the complete game [two starts ago] where he had the 120 [pitches], and then he pitched the 100-pitch game where he was in it the entire game. It's that time of the season where guys can get a blow if they can work it in, and I totally understand that."
For the second day in a row, Beckett declined to speak to the media about his injury.
"I don't know that there's any tests that necessarily have to get done," Valentine said. "They have a pretty good handle on it now. If he's going to get a shot or anything, maybe they'll do an MRI just so they know the right place to put it in."
If the Red Sox were in Game 7 of the World Series, would Beckett be able to pitch in his current condition?
"Oh, he'd pitch Game 1," Valentine said.
Beckett (4-7, 4.14 ERA) didn't show any signs of the injury in his most recent outing on Monday. The veteran was tagged for four runs in the first two frames against the Marlins, but he then settled in and didn't allow a hit over his final five innings of work.
The righty also missed a start earlier this season with a mild lat strain, but the club admitted at the time that he could have started had the Red Sox needed him to.
Late in Spring Training, Beckett was dealing with a thumb injury that is still presenting some difficulty, according to Valentine.
"It's an issue that will be dealt with here," Valentine said. "I don't know if they're going to do it during these two weeks. I would think we should. That was on the burner for the All-Star break. It's a situation."
By dealing with the thumb, Valentine was speculating that Beckett might get a shot.
As for the shoulder, the Red Sox aren't sure if Beckett will seek a second opinion.
"Oh, I don't know," Valentine said. "You'd have to ask him. Because we have the time, I bet you he'll do everything that's necessary to make sure he's going to be as good as he can be when he comes back."