Welbeck, a product of United's youth academy, moved to Arsenal on transfer deadline day for a reported £16million.
The 23-year-old England forward - scorer of two goals for his country in this week's Euro 2016 qualifying win in Switzerland - has netted 24 times in the last three seasons for the Red Devils since returning from a loan spell with Sunderland in 2010/11.
And Van Gaal has made it clear he thinks that is not up to scratch, drawing comparisons with United frontmen Robin van Persie, Wayne Rooney and Radamel Falcao, the prolific Colombia striker the Old Trafford outfit brought in on loan as Welbeck departed.
Speaking at a press conference on Thursday where United's two deadline day recruits, Falcao and Daley Blind, were being officially unveiled, Van Gaal said: "I have given all the players the chance to convince me of their qualities.
"With Danny Welbeck, yes, he was here from when he was nine.
"After coming back from Sunderland he has played three seasons for Manchester United.
"But he doesn't have the record of Van Persie or Rooney, and that is the standard.
"That is why we let him go - also because of Falcao, and the youngsters who can fit in."
Source : PA
Source: PA