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Rooney played with Ronaldo during a five-year spell at Old Trafford, winning three Premier League crowns and the Champions League.

The Portugal captain, who blasted in 118 goals in 292 games across all competitions in his first spell with United, could make his second debut as soon as Saturday against Newcastle United, who are the only club he managed a Premier League hat-trick against during his first term in Manchester.

Now managing Derby County, Rooney said he will not be watching his former team-mate's homecoming but predicts Ronaldo to have an "enormous impact" in his second spell.

"He's one of the best players in the world," Rooney told reporters ahead of Derby's clash with Birmingham City on Friday.

"Cristiano will know himself it's not going to be as simple or straightforward for him as it was in Spain or Italy.

"Cristiano's going to be a very big player, I think he's going to have big moments in the season and I'm sure he'll score a lot of goals.

"So I think United are finally ready to go and challenge for the title, and I really feel they have to this year."

Ronaldo missed a 3-0 victory over Azerbaijan with Portugal after being suspended following a caution against the Republic of Ireland for his last-minute goal celebration.

Indeed, in that game the 36-year-old broke Ali Daei's all-time men's international scoring record as he netted his 110th and 111th goals for the Selecao.

And Rooney pointed to that performance as a reason why his former colleague will thrive in the Premier League.

"We saw, in the game against Ireland, what he does," Rooney continued.

"[In the] last couple of minutes, he scores a couple of goals, great headers, so he can have a massive impact on the Premier League.

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