Van Persie, who has scored 10 goals for United this season, turned his right ankle in the closing stages of the 2-1 defeat to Swansea last Saturday and left the Liberty Stadium on crutches.
Scans have shown the injury is "not heavy", according to Van Gaal, but he still expects a relatively long spell out for the former Arsenal man.
The United boss said: "He is injured
He is the only injured player I have so we change (Michael) Carrick for Van Persie.
"He has an ankle problem
But I don't think it's very heavy but an ankle problem takes a long time
It's not one week or two weeks but we have to wait and see."
Source : PA
Source: PA