Swansea manager Michael Laudrup says the injury suffered by Michu in a 0-0 draw with Reading should not prove serious.
The striker played in the 1-1 draw against Swansea on Sunday but was not involved as the English Premier League leaders secured a 4-3 Boxing Day victory over Newcastle United.
United manager Sir Alex Ferguson has told MUTV the England international is struggling with a slight knee ligament problem and is facing two to three weeks out.
"Wayne had a volley yesterday towards the end of training and he's torn the ligament in the back of his knee," the Scot said.
"He could be out for two or three weeks which is unfortunate because I thought after his performance on Sunday he could be brilliant, you know you don't have two performances like Wayne did on Sunday.
"He was desperate to play and unfortunately couldn't risk him."
Ferguson was also without Ashley Young with a knock, while Danny Welbeck was ruled out prior to the game through illness.
United edged Newcastle at Old Trafford to move seven points clear at the top, with defender Nemanja Vidic an unused substitute due to soreness after the Swansea draw.