Jan Kirchhoff is facing a lengthy spell on the sidelines to add to David Moyes’ injury problems.
The former Bayern Munich man, who was this week linked with a move back to the Bundesliga with Hamburg, suffered a knee cartilage injury in the 1-0 defeat to Chelsea.
Kirchhoff, 26, will now see a specialist to determine exactly how long the midfielder, who has just returned from a hamstring injury, will be out for.
Moyes said: “We have lost Jan Kirchhoff with a tear in his lateral meniscus.
“We have had it scanned, he has not seen the specialist yet so I can’t give a definite on how long he will be out.
“But the lateral meniscus is worse than the medial meniscus so we don’t think he will be back too quickly.
“It is tough for Jan and hard on him because he has just come back from an injury and trained hard to get back.
“He had played a few games and trained, he was ready to come back.
“Jan probably had the best programme to recover, there is nothing you can do.”
Moyes’ midfield options have been hit hard in recent weeks with Lee Cattermole, Lynden Gooch, Paddy McNair all ruled out for long-term, while Duncan Watmore is also out for the season.
The Sunderland boss is sweating on the fitness of Victor Anichebe (thigh) and Steven Pienaar (calf) ahead of the Watford game, with Javier Manquillo also facing a fitness test.
Right-back Billy Jones is a big doubt after hobbling off in the second half of the defeat by Chelsea.
Moyes added: “Billy is a big doubt.
“He has a hip injury. I think the bigger thing for us is to try and get Victor and Steven back if we can.
“As we are getting a couple back in, we are losing a couple as well.
“We are trying to get that balance right as much as we can.
“It is a busy period, you will see by our squad that we are going to have to bring in some of the younger players.
“Sometimes that can be a good thing.
“Injuries are part and parcel of football. If ours were through training then I would question what we are doing.
“All of our injuries have come in games, serious knee injuries and cruciates.”
Should Jones and Manquillo miss out, then Donald Love will be drafted in at right-back.
Moyes added: “Donald did really well against Chelsea, he took the ball and played as well as he did against Manchester City on the opening day of the season.
“If he gets his opportunity, I have always thought he would be a steady player for Sunderland and still believe that.”
New of Kirchhoff’s injury comes at the same time as reports in Germany claiming Hamburg have been keeping tabs on him.
Kirchhoff joined Sunderland in January from Bayern Munich for a bargain £750,000, signing an 18-month deal on Wearside.
Versatile Kirchhoff - who played a key role in keeping Sunderland up last season - can play in either midfield or defence and German side Hamburg were understood to be keeping tabs on him.
Kirchhoff’s current deal runs out at the end of this season and Moyes has made it clear that talks with those players out of contract next summer are not on his agenda until Sunderland’s Premier League fate is known.