The 30-year-old, who has made 23 appearances in all competitions this season, suffered the injury in the first half of United's 1-0 win over Liverpool on Sunday.
Young now joins Antonio Valencia, Luke Shaw, Marcos Rojo and Phil Jones on the sidelines ahead of United's clash with Southampton on Saturday.
"Ashley Young has a severe groin injury, it shall be operated on, so he’s for a long time gone," van Gaal told MUTV.
"It’s unbelievable, but we’re not lucky with that. [Phil] Jones and [Bastian] Schweinsteiger cannot come back this weekend. You can count it and we have seven injuries."