Duncan Watmore was forced off 42 minutes into Saturday's 2-2 draw with Millwall, caretaker boss Robbie Stockdale's final game in charge
Sunderland forward Duncan Watmore has suffered a suspected cruciate knee ligament injury and is set to undergo surgery once scans have been taken.
The 23-year-old was substituted shortly before half-time in Saturday's 2-2 home draw against Millwall.
He has only played six times since returning from 10 months out with a torn ACL, also in his left knee.
"We have to get the clinical results to find out exactly what he has done," said new Sunderland boss Chris Coleman.
"We haven't got those yet, so everybody is looking at it and suspecting, but we don't know yet.
"Of course we are worried because of what happened before but until we get a definite, we can't really say what is what.
"Duncan worked so hard to get back. He's a real good player and an even better guy," he told BBC Sport.
Watmore was one of three Black Cats players forced to go off in the game against Millwall.
Marc Wilson and Jonny Williams will also miss Coleman's first game in charge on Tuesday night, against former Sunderland boss Steve Bruce's Aston Villa.
Source: bbc.com