Manchester United forward Cristiano Ronaldo is performing to the best of his abilities and could continue playing at the top level even at the age of 40, manager Ole Gunnar Solskjaer said.

The Portugal forward has scored four goals for United since returning to the Premier League club from Serie A's Juventus in August after 12 years away.

Solskjaer praised the physical fitness of the 36-year-old Ronaldo, who last week reclaimed the top spot among the world's highest-paid soccer players from Lionel Messi.

"I wouldn't be surprised if he was still playing at 40," Solskjaer said before Saturday's league game at home to Aston Villa. "The way he looks after himself, that's the key, that's the key to it all and, of course, some genes.

"He has put every single ounce of energy and effort into becoming the player he is."

The Norwegian manager said Ronaldo, who has won five Ballon d'Or awards for the best player in world football, is "hungry as ever" for laurels.

"His mentality is still absolutely spot on and that's just desire from inside of him," Solskjaer said. "He's going to keep going until his legs or even his head say, 'No that's it, I've given everything now.'"

United is third in the Premier League table with four wins and a draw but level on points with leaders Chelsea and Liverpool.