The left-handed-hitting center fielder was forced to miss his second straight game on Wednesday with the nagging injury. Making his temporary loss even more significant was that right fielder J.D. Drew was also out of the lineup with a sore left quad, sustained in the second inning of Tuesday's 1-0 victory vs. the Blue Jays.
That left Red Sox manager Terry Francona with an outfield bench of, well, nobody.
Left fielder Manny Ramirez, center fielder Coco Crisp and right field fill-in Brandon Moss started Wednesday's game with no backup. The only option would be shortstop Julio Lugo, who has appeared in 18 games in the outfield over the course of his career.
In fact, backup catcher Kevin Cash and utility infielder Jed Lowrie were the only reserves available off the bench for Wednesday's game.
Ellsbury could be back by Wednesday night.
"It feels better today," said Ellsbury. "I came in early, got treatment on it. I think it's something that should be good here shortly. I've had it before. In the past it was a little bit worse. This one was pretty aggravated when I did it. Just in the last few days, it has felt a lot better."
"He is not worse, but [it's] not good enough to send him out there," said Francona. "He's in getting treatment now. I'm glad he's not worse. I think you always fear that he'll wake up and be real sore. He's not, but he feels it, so we'll keep updating and checking on how he's doing."
As it is, the Red Sox are without their usual bench depth because first baseman Sean Casey (right hip flexor strain) and Alex Cora (sprained right elbow) remain on the disabled list.
There is good news on the Casey front, as he should be able to be activated on May 11, the first day he is eligible.
"He's actually doing real well," Francona said. "We actually got ourselves in a situation the other day where we didn't have very many healthy bodies. I think that when his time on the disabled list is up, he'll be back and ready to play. He's going to be out there hitting and doing a lot of stuff. I don't know how that happened where you hear a pop, and we all saw the way he ran into home, but he's really feeling pretty good. That is good news. In other circumstances, we may have been able to not DL him and hope for the best. In that situation, we had no chance to do that."
Cora is progressing with his throwing program, but there's no set timetable for his return.
"I think the elbow is doing pretty well," Francona said. "The thing is, even when you're down for even a week or 10 days or two weeks, you still have to build up your arm strength or you're going to ask for problems, even with a completely healthy young kid.
"They're just in the process of getting him built back up and then when he feels comfortable, which I don't think is too far away, he'll start taking ground balls, turning double plays and then eventually he can go into the outfield and make relay throws. I think that's the biggest thing AC wants to do, make some throws from a cutoff position to make sure he can turn it loose."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.