Ellsbury will be sent to Los Angeles on Saturday to receive an MRI after leaving the Red Sox' 10-9 loss at Texas on Friday night with soreness in his left side, the same area that saw him break multiple ribs earlier in the year.
The speedy center fielder, who was originally hurt in a collision with third baseman Adrian Beltre back in April, hurt himself on the opening play of the game Friday after colliding with Texas pitcher Tommy Hunter at first base and then rolling onto his left side.
Ellsbury's left arm was caught under him as he landed and he instantly knew something was amiss.
"Just felt a sharp pain and tried to play through it," said Ellsbury, who came out in the bottom of the fourth after reaching on an error and scoring in the top half.
The 26-year-old said he wanted to stay in the game but manager Terry Francona thought it would be best to take every precaution.
"There's certainly some concern there," Francona said of the injury.
While the Red Sox hold their breath on Ellsbury, they have to wait and see on two others who struggled through the four-hour, 10-minute affair in temperatures well into the 90s.
Jed Lowrie, who started at second base and homered in the second, was removed in the ninth with what Francona called "heat exhaustion."
"He was hurting. He was going down in a hurry," Francona said.
Perhaps of even greater import is the condition of catcher Victor Martinez, who fouled a ball off his left foot in the same area he has bruised in the past. Martinez remained in the game but was in obvious pain.
"Same spot. He's pretty sore," Francona added in a postgame interview that like so many others focused on a string of health-related issues. "He's a pretty tough kid but we'll see how he shows up [Saturday]."