The 23-year-old 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.