Sunderland rocked by injuries to Watmore and Williams
The forward will undergo exploratory surgery on Thursday but the club is fearing the worst with the forward set to be ruled out for the season.
Chris Coleman says Watmore will be a “huge loss”, with the Sunderland boss also confirming midfielder Jonny Williams needs surgery on his shoulder and could be out anywhere between one and three months.
Watmore’s knee injury is a devastating, the 23-year-old was substituted shortly before half-time in the draw with Millwall.
Watmore only returned on September 30, after 10 months out with a torn anterior cruciate ligament in the same left knee that has been damaged this time.
Coleman, speaking after the 2-1 defeat to Aston Villa, said: “We will know more after Thursday’s surgery, in terms of a timeline that is when we will have the news.
“It is such a shame for Duncan. It is a shame for us but an even bigger shame for Duncan because he has worked so hard to get back.
“He is a huge loss for us because he has huge potential. We don’t yet know how long that will be but we anticipate that it is not the news we would hope for.
“I had a chat with Duncan on Sunday and of course he is down and disappointed, he is a great lad.”
Williams went off after just 16 minutes off the Millwall draw and he also needs surgery on a damaged shoulder.
Coleman added: “Jonny will have surgery on his shoulder. It could be a month, three months depending. That is another big blow.”