"He didn't feel it during his side [session]," Valentine said. "It was in the training room when they tested him. He's not quite recovering on a five-man rotation right now."
Valentine said the injury is "nothing serious," though the Red Sox likely will seek assurance of that in additional testing.
"He'll be pitching again, that's for sure," Valentine said.
Beckett last missed a start May 5 with what was described at the time as soreness in his right lat, the large muscle in the back of the shoulder. He sparked a tempest when it was reported that he had gone golfing the day after Valentine had said he was unable to take his turn. Beckett added to the controversy when he insisted he did not have to answer for his behavior.
Upon returning to the rotation on May 10, Beckett was hammered for seven runs in 2 1/3 innings in an 8-3 loss to the Cleveland Indians, and was loudly booed. Since then, he has posted a 2.68 ERA in six starts, though the Red Sox have dropped the past three.
There was little to indicate Monday night in Miami that Beckett was having arm issues. His fastball velocity averaged around 91 mph and he touched 93. Beckett threw all of his pitches, though he relied heavily on his two-seam fastballs while throwing far fewer cutters than he had in his previous start against the Baltimore Orioles.
The 26-year-old Morales has a 3.04 ERA in 23 2/3 innings out of the bullpen this season. He has 15 career starts, his last coming in 2009 with the Colorado Rockies.