Rooney picked up the strain in training this week and has been left out of the squad to face Liverpool but van Gaal hopes to have him back in time for Southampton next weekend.
With so few options up front, having Rooney sidelined for any length of time is a concern, even if his current form for the club is poor.
Speaking about Rooney’s injury, van Gaal said: “It’s not too serious but we cannot take a risk.”
Michael Carrick takes the captain’s armband for United today against Liverpool in Rooney’s place.
This situation is one which United fans knew would happen at some point this year because Rooney always tends to pick up niggles and that’s why there is some concern over United’s attacking options, especially with James Wilson who is set to leave the club on loan.