Cristiano Ronaldo is a "goal machine" who will remain competitive until the day he retires, former Italy and Juventus defender Gianluca Zambrotta has told Stats Perform.

Five-time Ballon d'Or winner Ronaldo has reportedly asked to leave Manchester United if an offer is made after just one season back at Old Trafford.

The Portugal international has yet to return to training with United and is not part of their tour of Thailand and Australia, which has been put down to family reasons.

Ronaldo scored a team-leading 24 goals in all competitions last season, but his homecoming did not go to plan as United registered their lowest Premier League points tally.

United endured a fifth successive season without silverware and finished down in sixth, meaning no Champions League football in the 2022-23 campaign.

That is said to be the reason why Ronaldo is seeking a new challenge, and Zambrotta has backed the 37-year-old to continue firing wherever he plies his trade.

"Ronaldo is a player who makes a difference at the moment. We have seen it. The numbers say this," Zambrotta said. 

"Which team can be right for him? I don't know. Definitely a team fighting for the Champions League, a team fighting for many goals.

"Because I am sure that he always wants to be competitive, until he retires. And I still think Ronaldo is a goal machine any team he plays for."

Ronaldo is the Champions League's all-time leading scorer with 140 goals and has won the competition four times with Real Madrid and once with United in his previous spell.

He scored six goals in last season's competition but could not help United further than the last-16 stage, where they were eliminated over two legs by Atletico Madrid.

However, while United endured a dreadful campaign under Ole Gunnar Solskjaer and then Ralf Rangnick, Zambrotta does not feel Ronaldo was to blame.

"Let's say that he also needs team-mates," said Zambrotta, who represented the likes of Juve, Barcelona and Milan, while helping Italy to World Cup glory in 2006. 

"Even when Ronaldo was criticised at Juve, and they weren't able to move forward in the Champions League, he has always made the difference from a numerical point of view. 

"The data, the numbers say this. You can't think of going against the numbers. And then you may like him, or you may not like him – this is another kind of speech.

"But he has always been a player who has always made the difference from that point of view."

Ronaldo is not the only big-name attacker potentially on the move this window, with Robert Lewandowski also eager to move on from Bayern Munich.

Lewandowski scored 50 goals in 46 games in all competitions last season in his second-best scoring campaign across eight years with Bayern, behind the 55 netted in 2019-20.

The prolific striker has less than 12 months to run on his contract and has made no secret of his desire to leave, with Barcelona this week confirming an offer has been tabled.

It could lead to a bitter split between Bayern and Lewandowski but, like with Ronaldo, Zambrotta can understand why the player wants out.

"I have always been used to not judging anyone because I don't know the personal stories and current events of each case," he said. 

"Everyone can find themselves in a certain moment of their life or career in which they need to change because they need new motivations, new stimuli. 

"He needs a change of scenery, to see new faces, to stay in another stadium, to meet new mates. 

"Maybe at that moment, it was perhaps the right one for a player, so it doesn't affect me and most likely [Lewandowski] wants to change the air. That's all."