Wayne Rooney has backed Cristiano Ronaldo to have an "enormous" impact at Manchester United.

Ronaldo re-signed for the Red Devils in the final stages of the summer transfer window after making it clear that he wanted to leave Juventus one year before the end of his contract.

Rooney starred alongside the Portugal international during his time at Old Trafford, and the now Derby County boss has said that the 36-year-old's arrival will help Ole Gunnar Solskjaer's side challenge for the 2021-22 Premier League title.

"What impact will Ronaldo have? An enormous one, I think he's still one of the best players in the world," Rooney told talkSPORT. "I think we've seen in the game against Ireland that's what he does, last couple of minutes he scores two goals, two great headers, so he can have a massive impact on this league.

"I think United are finally ready to go and challenge for the title, and I really feel they have to this year. [Ronaldo's] going to be a big player, I think he'll have very big moments in the season and I'm sure he'll score a lot of goals."

"I think his mentality will rub off on the younger players especially, so I think it's great timing. I think Cristiano will know himself it's not going to be as simple or as straightforward as it was in Spain or in Italy for him.

"It's a physical league, he knows, he's played in it, so I'm sure he's prepared and he's ready to go and score goals again for Manchester United."

Ronaldo has been training with his new teammates this week after being allowed to leave international duty early, so he is is contention to make his second debut for the club against Newcastle United on Saturday.