Sam McQueen is set for a lengthy spell on the sidelines after tearing a cruciate ligament in his knee, Tony Pulis has revealed.
The Southampton loan man was stretchered off in Wednesday's 1-0 victory over Crystal Palace.
And the Boro manager has revealed the extent of the damage.
"Sam's news is not good," Pulis said in his pre-match media conference on Friday.
"He's done his cruciate so I don't think we'll have him for a very long time.
"We're talking to Southampton now to see if he'll stay here to recover or if he'll return to his mother club.
"It's a real disappointment because Sam's not had the opportunities he would have liked, but he is a great character and he has been fantastic in the dressing room."
McQueen is one of a number of players who could miss out on Saturday's match against Stoke City, including fellow full-back Ryan Shotton, while Daniel Ayala misses out having picked up five yellow cards this season.
"I don't think Shotts will be ready in time for Saturday's game," Pulis said.
"With the bug we had earlier in the week, the doctor has had a few in earlier today to see how they are. We'll go through that and we'll pick the team from there.
"There will be a few changes, because of injuries and suspension. We've got a question mark over Dael Fry so that's four defensive players we could be without, but you have to put up with it.
"The ones we call on we want them to go out there and give it their best."