In the Dutchman's first competitive game in charge, the Red Devils were beaten 2-1 at home to Swansea City.
Irwin, who made more than 360 appearances for United, told reporters: "We have to trust him because he is an experienced manager. He has won things everywhere he has gone.
"It was a disappointing result and a poor performance but going forward, I think we might need a little bit of patience for the manager to get his ideas across."
The 20-time league title winners travel to Sunderland on Sunday hoping to make amends for last week's performance.