Seb Larsson is expected to miss the remainder of 2015 with the knee injury which ruled him out of facing former club Arsenal.
Larsson travelled to see a specialist in Barcelona last week over a medial ligament problem which had been affecting him for several weeks.
The Swedish international was immediately sidelined for Saturday’s clash at the Emirates and fitted with a knee brace until the extent of the damage was known.
But it is understood that Larsson is facing around a month on the treatment table, to leave Sam Allardyce without the services of the midfielder throughout the busy festive programme.
It’s a blow to both Allardyce and Larsson, himself, after last year’s Player of the Season had just won back his place in the starting XI.
However, while Larsson’s injury has a clear timescale, fellow midfielder Lee Cattermole’s problem is a concerning one for Sunderland.
Cattermole suffered a recurrence of his back injury in training on the eve of the Arsenal game, despite the Teessider visiting a specialist during the last international break about the issue.
Sunderland had hoped that the subsequent treatment would solve the problem, but Cattermole continues to be a troubled by an issue stemming from a disc in his back pressing against the nerve.
Cattermole is highly unlikely to face Watford this weekend, but Allardyce will be able to call upon Jermain Defoe, who also missed the Arsenal clash.
The hamstring injury suffered by Defoe against Stoke 10 days ago was only a minor one and he is back in full training ahead of Saturday’s visit of the Hornets.
“Jermain will be back, I don’t know about the other two,” said Allardyce after the 3-1 loss at Arsenal.
Meanwhile, Jack Rodwell is hopeful of enjoying an injury-free run after travelling to Germany for treatment on his hamstring problem last week.
Rodwell has endured a series of niggles this season, with the £10million midfielder making his first appearance in a month as a substitute at the Emirates.
The 24-year-old had only endured one day of full training with his team-mates prior to the game, but the injuries to Larsson and Cattermole prompted Allardyce to use the ex-Manchester City man.