Sunderland boss Steve Bruce is sweating on the fitness of midfield enforcer Lee Cattermole.
The former Wigan man picked up a knee injury during the Black Cats' contentious 1-0 victory over Liverpool on Saturday.
He was forced from the field on a stretcher and is now set to undergo scans in an effort to determine the severity of the problem.
Sunderland fear the England U21 international may have damaged medial ligaments and could be set for a lengthy spell on the sidelines.
Such a problem would likely rule Cattermole out of action until well into the New Year.
While Bruce remains uncertain as to how long he will be without the services of his summer signing, he has received better news concerning the fitness of Kenwyne Jones.
The talismanic striker was also forced off against the Reds, with lengthy treatment required on an ankle complaint.
He is set to sit out the weekend trip to Birmingham as a result, but it has been confirmed that he has suffered no break.
"Fortunately the X-rays showed no break, but Kenwyne's still very sore and I'd rate him as 50/50 for the Birmingham game," Bruce told the club's official website.
Victory over Liverpool on Saturday moved the ever-improving Black Cats into seventh spot in the Premier League table.
With the 2009/10 campaign promising much, club captain Lorik Cana is hoping both Cattermole and Jones will be back in action sooner rather than later.
"They are big players for us and I hope they don't miss many games," he told the Sunderland Echo.
"We need everyone because we don't have 24 or 25 players like the top teams."