Several ex-players at United have spoken about the appointment of Dutchman ten Hag, who was officially announced on Thursday.
He will take over in the summer, and Rooney believes that the highly rated coach will need time to get it right.
Rooney said at a Premier League Hall of Fame event: "It's a massive job and it's good to see they have now put a manager in that looks like he'll get a bit of time.
"I'm sure they are going to let him rebuild the squad because that needs to happen.
"Fans have to be a little bit patient with him because it won't change overnight or as quick as you'd like.
"They need to be a little bit patient as long as they see progression, players working and playing for the shirt with hopefully a look at trying to challenge for the title in the next three years. That is where United fans have to be realistic and I am sure that's what the club are hoping for."
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