According to manager Terry Francona, Ellsbury complained of pain in his left side -- in a spot different from where he'd cracked four ribs in an April 11 collision with third baseman Adrian Beltre -- after a session in the batting cage with hitting coach Dave Magadan.
The team was sufficiently concerned that Ellsbury was administered both X-rays and a CT scan and was examined by Tampa Bay's team doctor, and Boston abandoned plans to designate McDonald for assignment. Instead, reliever Scott Atchison was designated, a procedural move that will lead Atchison to being optioned back to Pawtucket on Thursday.
"Everything looks pretty good," said Francona, though he was unable to identify Ellsbury's precise injury or its exact location.
This much is certain: "I can't imagine us playing him [Wednesday]," said Francona, who already had given Ellsbury the day off after he'd played three straight games since coming off the disabled list Saturday.
"I also think he got a little nervous. I understand that. We certainly want to look after him and sit down and talk this through."
This latest physical setback for Ellsbury came a short time after Francona had announced that the team had reversed its off-season plan and intended to have Ellsbury be the everyday centerfielder, with Cameron playing in left.
Cameron marked his return to the Red Sox lineup Tuesday night, after an absence of five weeks and 34 games due to an abdominal tear, by playing center field against the Tampa Bay Rays, with Ellsbury getting the night off even before he complained of renewed pain.
Cameron has started in left field just once as a professional, and that was with Triple-A Pawtucket earlier this month while on rehab assignment. He has never played left field in the big leagues.
"I think that's probably the right thing to do, at least for the moment, in the near future," Francona had said before the game. "I talked to both of them, and both were pretty good about it. Both said, whatever we need to do. I said we can't play Cam every day yet. Hopefully we get to that point, [but] we need to be aware of what he's gone through and keep him going forward. Because we have [Jeremy] Hermida, it's kind of a natural back and forth."
Cameron was supposed to give the Sox an upgrade in their outfield defense in center -- he led the National League in a number of defensive categories last season for Milwaukee -- with the overall outfield defense also improving with Ellsbury replacing free agent Jason Bay in left.
Red Sox GM Theo Epstein had indicated during the winter that the move was hardly a permanent one and could even change at some point this season, but it's unlikely anyone anticipated it happening this soon.
On the face of it, it would seem to signal the concern that Cameron's injury could be problematic later in the season -- he has told reporters he expects he'll need surgery at some point -- and at the least the 37-year-old outfielder will need to be rested more than originally planned. Francona acknowledged that the shift to left may save some wear and tear.
"I would say uncharted waters for me," Cameron said regarding what level of discomfort he expects to be dealing with. "We'll find out. The times I've played I've gotten better and better. It's just a matter of how much the body is able to withstand.
"It's almost like spring training for me a little bit, as far as going out and playing, regroup do it again the next day and the next day. I'm sure there will be a little more of an exhaustion period here."
If Cameron has an issue with the move to left, he's keeping his own counsel, though he did mention the unfamiliarity of the position and the adjustment that will entail, especially learning the Green Monster in Fenway. Ellsbury had played left as a rookie in 2007.
"I don't think I'll have any problem with that," he said of the move, noting that the Sox had raised the possibility he'd be playing left when he first signed.
"As to what I have to do, I feel pretty good. We'll see how it goes out. I can't really be concerned with that. I just want to get a chance to run around and play the game the best I can possibly play it. If that's the case [playing left field], that's what it is."
The shift of Cameron to left suggests the Sox may indeed look to platoon him with the left-handed hitting Hermida, which certainly was not Cameron's intention when he signed and declared himself an everyday player. Hermida has 24 RBIs in a part-time role, 15 of those RBIs coming with two outs and runners in scoring position.
Francona indicated Hermida, who played left field Tuesday night, will get playing time while Ellsbury and Cameron work their way back into daily playing status. Ellsbury has a single in 14 at-bats in the three games he has played since his return from a hairline fracture in four ribs. He went 0 for 5 Monday night, grounding out in all five of his plate appearances.
Now, with Ellsbury's condition again uncertain, Cameron is likely to remain in center indefinitely.