Ellsbury, who missed 37 games with hairline fractures in four ribs on his left side, was activated Saturday. He has appeared in three games since, going 1-for-14 with a walk and two RBIs. He has also experienced pain in the area of the injury.
"He's been examined," said Red Sox manager Terry Francona. "I mean, we have looked at the pictures. All the medical people are trying to look at everything. He's really tender. [Team medical director] Dr. [Tom] Gill will be here tonight. We'll try to meet again. We got to get to the bottom of this. We're not at the bottom of it. We don't want to rush into putting him on the DL. At the same time, he's not ready to play because of the way he feels. ... If we have to get more opinions, we'll do what we think is necessary.
"I think what I'm saying is that the pain was in a different area. It's got to be related. Whether he was compensating with the swing, that's what we're trying to figure out, but it's all related.
"It really doesn't feel better."
After Thursday's 4-3 loss to the Royals, Francona said Ellsbury would undergo further testing Friday morning.
"He'll be examined again [Friday], about 11 [a.m. ET]," Francona said. "I spoke to [Gill] and some of the medical people ... trying to figure out the best way to go through this. He'll have one more examination in the morning. Hopefully we'll have some more insight on what's the best way to go with this."
Cameron, who missed 34 games with a lower abdominal strain, was activated Tuesday. He was not in the lineup after playing in back-to-back games Tuesday and Wednesday, going 2-for-8 against the Rays.
"He's a little sore today," Francona said of Cameron. "I think we knew that. Getting in at 4:35 in the morning, playing on turf, there's a lot of things we knew we'd have to, I think I used the words 'finesse it' the other night, build in some rest for him, which we'll do."
With Ellsbury and Cameron out, Jeremy Hermida started in left and Bill Hall got the start in center against Kansas City.
Martinez has been nursing a sore left big toe, after leaving Monday's game against the Rays.
"It's hard to tell this afternoon," Francona said. "We got here so late just because of our arrival. He did a lot [Wednesday] -- BP, he did some throwing, he did some receiving. I think the hope is he'll run around the outfield today a little bit. We'd really like to catch him [Friday] because of [Tim Wakefield]. If he's not ready, he won't do it. I think he'll be ready to catch."
With Daisuke Matsuzaka pitching Thursday, Jason Varitek was behind the plate and hitting eighth.
Infielder Jed Lowrie, who has been out since Spring Training with mononucleosis, was in the clubhouse before the game after arriving in Boston on Wednesday night to be examined by the medical staff Thursday.
"He was looked at by various medical people just to eliminate I think some of the doubts maybe that he has, some of the fatigue he's feeling," Francona said. "I think he wants to know that he's OK, so it's maybe a normal tired instead of something that's eating at him that was missed or something like that. So [we're] just trying to eliminate some of the doubts in his mind."
Lowrie said he feels much better and is progressing with the illness. He was scheduled to head back to Fort Myers, Fla., where he has been since Spring Training, on Thursday.
"Fort Myers is where I need to be right now," he said.