The Cameroon international has made a flying start to life at Old Trafford, scoring five goals in ten appearances, including a brace in the 4-2 win over Brighton and a crucial opener in United's first win at Anfield since 2016.
"I wasn't sure about the signing when it happened," Rooney admitted on The Wayne Rooney Show. "But what Bryan's shown is that he belongs here. His energy, his work rate, his desire - that's what you need to be a Manchester United player. He's been consistent, and when you keep getting into the right positions, the goals come."
Manager Ruben Amorim also praised Mbeumo after the Brighton victory, calling him "a working machine" and highlighting his partnership with Amad Diallo.
"They're fast, they swap positions, and they're hard to defend against," said Amorim. "He's improving every week."
Rooney also backed fellow new signing Matheus Cunha to build on his first goal for the club.
"You could see how much it meant to him," said Rooney. "He's been playing well, and that goal will give him confidence to kick on."
Captain Bruno Fernandes echoed Rooney's praise, saying United's new recruits have brought "big personalities as well as big talent."
With three consecutive league wins lifting the mood at Old Trafford, Rooney believes Mbeumo stands out as United's top addition.
"He's full of confidence right now," he said. "He's been brilliant - definitely the best signing for United this season."