Jack Ross has revealed that Chris Maguire is set to be absent for up to eight weeks after suffering a fractured fibula.
The popular forward was withdrawn in the early stages of the Black Cats' win over Gillingham on Tuesday evening, having been recalled to the starting line-up by Ross.
But the Sunderland boss has now confirmed that the Scot is set to miss the next six to eight weeks - although the club do not anticipate that he will require surgery which would have increased his time on the sidelines.
Nonetheless, Ross admits it is blow to lose the 30-year-old, who had looked to be rediscovering his early season form in recent outings.
"Chris has a fractured fibula, so he will be out for between six and eight weeks," he said.
"Hopefully he is more towards the six week period of that.
"He has another appointment next week to confirm he doesn't need surgery but we don't think that will be the case.
"It's a blow for him and a blow for us as he's been important in terms of goals and assists.
"Over the last game and a bit he had looked as if he was back at his levels which had made him so productive for us at the start of the season, so it's another thing for us to deal with it but it's just what happens through the course of the season."
And while Ross admits that while Maguire is frustrated that he will have to sit out a large chunk of the season run-in, he is already setting targets for a return and is keen to play his part as Sunderland eye success on all fronts.
"He's frustrated because he just wants to play all the time, that's how Chris is as a guy," he added.
"He knows he's going to be out and not be able to do much for four to six weeks anyway, and then obviously he wants to contribute on the park.
"He wants to play a part in things and knows what lies ahead of us as well.
"We've got an exciting end to the season in two competitions and it could be very enjoyable.
"He's already setting targets for when he thinks he could be back, so he's okay just now.
"We've given him some time off that will help him in terms of clearing his head.
"I would prefer not to have it, but we just have to find a solution to keep winning games."