Young picked up the injury in United's 1-1 draw with Stoke City on New Year's Day and did not feature in the 2-0 FA Cup third round win over Yeovil Town last Sunday.
And manager Van Gaal has revealed that the former Aston Villa man is poised for a lengthy spell on the sidelines.
Speaking at his press conference prior to Sunday's Premier League clash with Southampton, Van Gaal said: "Ashley Young - four weeks to six weeks, I think.
"It's always difficult to predict in a muscle disease."
Versatile midfielder Daley Blind, who has not featured since sustaining a knee injury in Netherlands' 6-0 win over Latvia in November, could make his return, leaving Young as United's only absentee.
"I'm very happy with your question because we have only one player injured," Van Gaal responded when asked for an update on United's injury situation.
"Can you believe it? Only one. I have to select for the first team in my career at Manchester United the eleven and the eighteen players."
United signed goalkeeper Victor Valdes on Thursday, the former Barcelona stopper agreeing the move after training with the Old Trafford club since October to aid his recovery from a serious knee injury suffered last year.
Van Gaal indicated that Valdes - who will initially serve as a back-up to fellow Spaniard David de Gea - has proven his quality during that time frame.
"When you invite Victor Valdes to have his rehabilitation here at Manchester United, you do that for him, but also for yourself," he added.
"I had the chance to see him and his behaviour in rehabilitation but also if his quality was still the same.
"He has confirmed both and that is very important for me and also for him because when I am satisfied then mostly the players are satisfied."
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