The England international joined the Gunners on transfer deadline day last Monday for a reported £16m fee following United's loan capture of Radamel Falcao.
In this afternoon's press conference, Van Gaal said: "When you ask me about Welbeck - he has been here since he is nine and he spent three years here after being at Sunderland, but he doesn't have the record of Van Persie or Rooney. That is the standard. That's why we let him go, because of Falcao and also the youngsters we have to come in."
Falcao could make his United debut at Old Trafford this Sunday against Queens Park Rangers.