Television pictures showed him hobbling on crutches, with a protective boot on his left foot.
The Belgian midfielder came on as a substitute for Ander Herrera in United's shock 2-1 home defeat by Swansea City on Saturday.
Reports had suggested he would be sold this month but unless that injury proves less serious than expected, that would now seem unlikely.
In a disappointing first season after signing from Everton, the 26-year-old started only 12 Premier League games and failed to score a goal.
United already have midfielders Michael Carrick, Tom Cleverley and Antonio Valencia suffering from injury, as well as defenders Jonny Evans and Luke Shaw and strikers Danny Welbeck and Robin van Persie.
New manager Louis van Gaal has been attempting to strengthen the squad and the club reported on Tuesday having agreed a fee with Sporting Lisbon for the Argentine international defender Marcos Rojo.