Sunderland midfielder Duncan Watmore may need a second operation on his injured knee after suffering discomfort on his return to light training.
Watmore, 23, who was effectively ruled out for the rest of the season with anterior cruciate ligament damage in December last year, may now face a further spell on the sidelines.
He is due to see a specialist this week to discover whether or not further surgery is required.
Manager David Moyes said: "Duncan Watmore might be needing a little bit of work done on his knee. Obviously, he's not back, but he might be needing a little bit more surgery on his knee.
"He's been feeling it -- he's been running a little bit -- so he'll see the surgeon this week."
The England under-21 international has been one of the few bright sparks for the Black Cats this season as they attempt to stave off relegation for the fifth successive year.
His pace and direct approach have been badly missed as Sunderland have struggled to provide support to leading scorer Jermain Defoe in recent months.
Meanwhile, Moyes is refusing to accept that relegation is inevitable as he prepares for a do-or-die derby trip to Middlesbrough.
The Black Cats head for Teesside on Wednesday evening 12 points adrift of safety with only 18 left to play for, and knowing defeat could prove all but fatal to their hopes of avoiding relegation from the Premier League.
However, he remains convinced that, despite the odds being stacked heavily against them, they have a genuine a chance of pulling off perhaps their greatest escape yet.
He said: "There's still a chance and while there's a chance, we're going to try to take it. That chance comes at Middlesbrough on Wednesday and we will try to take our chance.
"If we can take our chance there, then we will have another chance against Bournemouth on Saturday.
"We're not stupid, but we are in there fighting and we still think we have got a good chance.
Moyes has found himself in the firing line from sections of Sunderland's supporters in recent months as the club's season has limped towards its conclusion, and even striker Fabio Borini appeared to be making a point when he sprinted towards the bench to celebrate his late equaliser in the 2-2 draw with West Ham last time out in front of, rather than with, his manager.
But Moyes said: "I was thrilled about his celebration because he had scored us a goal.
"But I think I'd have really liked him to have gone and picked the ball out of the net and put it in the centre circle because we needed to score a winner to get three points.
"I don't think it's nice when you're not winning most weeks, and that's the thing. You want to win, you come into football to win. At the moment, we are not winning, so that's the hard bit to take."