Sunderland boss Gus Poyet admits Steven Fletcher's Achilles injury is a cause for concern as surgery may be required.
The Scottish striker has been nursing a complaint for several weeks and was forced to sit out Saturday's 3-0 derby victory over Newcastle United as a result.
Poyet has now revealed that the problem is worse than first feared and knows he must make a tough call on whether to send a key man under the knife or hope that a period of rest brings the 26-year-old back into his plans.
He told the Sunderland Echo: "It's affecting him more than I thought. I'm a bit worried about it.
"We're analysing it with the medical staff. But we're going to have to make a decision one way or the other.
"We'll have to look at the games ahead and the chance of rest in between, but we'll need to make a strong decision."
With it possible that Fletcher could be the sidelined for some time, Poyet admits that Sheffield Wednesday's hopes of taking Connor Wickham back to Hillsborough on loan are likely to be scuppered.
He added: "It probably makes it more likely that Connor will stay for longer.
"Connor has got plenty of admirers, you can't imagine how many. But he wasn't going anywhere on loan before the transfer deadline.
"He's part of the squad and he's done very well at Sheffield. That's why he's got so many options. But at the same time, he's our player.
"He's been on the bench for the last two games and he would have come on against Stoke if Phil Bardsley hadn't got injured, which changed my plans.
"The decision at the moment is that he's our player and we need him here."