Rooney missed United's 4-3 win over Newcastle on Boxing Day and Red Devils manager Sir Alex Ferguson confirmed the striker will miss the remainder of the festive programme.
"It was a volley towards the end of training," said Ferguson. "He did a ligament at the back of his knee and could be out for two to three weeks."
He also faces a race against time to face Liverpool at Old Trafford on January 13.
And for Ferguson it was a particularly disappointing given Rooney's poor performance at Swansea on Sunday.
"After his performance on Sunday I think he would have been brilliant today. You don't have two performances like he did on Sunday," added Ferguson.
"Unfortunately we couldn't risk him."
It was part of a negative fitness bulletin from the United boss, even if details of exact injuries were sketchy.