Middlesbrough midfielder Julio Arca could be out of action until December after scans confirmed medial ligament damage.
The 26-year-old Argentinian is facing at least two months on the sidelines with the knee problem he suffered against former club Sunderland on Saturday.
That will come as a huge blow to manager Gareth Southgate, although he was at least able to comfort himself with the news that strikers Tuncay Sanli and Mido could play against next weekend after tests on their respective injuries revealed no lasting damage.
Southgate told the club's official website: 'We have had some more positive news this afternoon.
'Both Mido and Tuncay have gone for scans and there's no significant damage, and both have a chance of playing on Sunday.
'However, the injury to Julio looks more serious. It's the medial ligament in his knee and it looks like it will be at least two months.'
Arca will be a big miss for the Teessiders with he and Brazilian Fabio Rochemback having formed a formidable central midfield partnership this season.
He hobbled off the field after extensive treatment 25 minutes into Saturday's 2-2 Premier League draw, but then had to be carried on a stretcher to the dressing room.
Southgate later admitted the fear was that he had damaged his medial ligament, and that proved to be the case.
The manager at least knows he has cover in that position with skipper George Boateng, who has played much of his football this season wide on the right rather than in his favoured central position, starting on the bench at the weekend to accommodate new signing Gary O'Neil.
Youngster Lee Cattermole was alongside him, giving Southgate another genuine option.
However, the same cannot be said in the striking department, and that makes the prognoses for Tuncay and Mido all the more encouraging.
The Turkey international took a nasty kick on the shin, while the Egyptian damaged a hamstring before the break, but played on until the 58th minute.
With Jeremie Aliadiere already out for a month with his own hamstring problem, that left Southgate with only one fit recognised frontman, Dong-Gook Lee, who has scored only once since arriving at the club in January.
The manager admitted after the game he would have to look at out-of-contract players, and he may still have to turn to 20-year-old Tom Craddock and Ben Hutchinson, 19, for Wednesday's Carling Cup third round trip to Tottenham.
But the news that Tuncay and Mido could be back as early as next weekend means the problem is likely to be temporary.
That will come as a boost with Mido in particular having made a major impact since his £6.8million arrival from Spurs.
Southgate was ready to replace him before the break with Cattermole stripped on the sidelines, but the Egyptian was determined to play on.
Southgate said: 'He epitomises the spirit in the dressing room. It was clear that he had a problem with his hamstring.
'We assumed we needed to get him off, but he wanted to stay on. We knew he would be limited by it and in the end, sometimes you have to save him from himself because there is always the risk of doing more damage in that situation.
'I think he realised that it was too limiting for him as the game went on.
'But you can only admire his desire to stay out there and battle for his team-mates.'