Argentina boss Gerardo Martino has reiterated he will always vote for Lionel Messi in the Ballon d'Or - although he admits it is impossible for him to make a truly objective judgement on the matter.

Messi has been named world player of the year on four occasions, but it is expected he will be pipped for the 2014 award by Cristiano Ronaldo, the 2008 and 2013 winner.

The way Ronaldo's season has gone so far suggests that would be just rewards for the Portugal star, who is set to go head-to-head with Messi on the pitch at Old Trafford on Tuesday when the pair's national teams meet there in a friendly.

Martino has already made it clear where his loyalties lie, though, and did so again at his pre-match press conference on Monday.

"You can't take out of the decision-making process the fact I am Argentinian and my heart lies with Messi," said Martino, who also managed the player at Barcelona last term and then took over the Argentina post in August.

"I'm probably not the best person to be asked to judge the Ballon d'Or because I cannot separate sentiment from my decision - I'm not the most objective person.

"Whatever kind of season Messi might have had from a playing point of view, I would still vote for him."

Martino, who has started Messi on the right in Argentina's last two games, regards Ronaldo as "world-class" but is not planning a man-marking job on him.

"It is not a case of giving special, individual attention when marking, but when you come up against a world-class player, you pay attention to him as a team," Martino said.

"It is about trying to reduce the space in which he can operate."

At the World Cup in Brazil over the summer, Ronaldo and Portugal failed to get out of their group while Messi and Argentina were beaten finalists.

Since then, 2014-15 has seen Ronaldo notch a total of 25 goals for his country and club Real Madrid - with whom he won last season's Champions League - combined, while Messi has 14.

Portugal boss Fernando Santos feels each forward, both captain of their countries, is a genius, but has no doubt it is his skipper who deserves the Ballon d'Or this time around.

"I voted for Cristiano because I think during the season he has been the best player in the world," said Santos, appointed to his current job in September.

"We all know Messi is a good player and in my opinion they are both genius players, but I made my choice."

There was also praise on Monday for Ronaldo from Portugal defender Ricardo Carvalho.

"For us Portuguese, everyone is expecting that Cristiano will win his third Ballon d'Or," the Monaco and former Chelsea and Real centre-back said.

"I think he is the best. He always wants more. He is almost perfect - strong, good in the air, and always fit."

Ronaldo, the former Manchester United man returning to his old ground, and Carvalho are just two of a host players of the Barclays Premier League past and present in line to feature in Tuesday's contest.

Among them is Nani, the Portugal winger so often derided for his performances for United, who is currently on loan from them in his homeland with Sporting Lisbon.

Santos said: "It is very important that Nani is in Portugal, because he is playing regularly. It is good for us because that way he improves his form."

Another is current Red Devil Angel di Maria, the Argentina winger who cost a British record fee of £59.7million when he switched from Real to United in August.

Di Maria made a very positive impression in his first few games for his new club, but has been more quiet of late.

And Martino feels that sort of dip was perhaps inevitable.

"Angel is one of the very top players in world football and I think his start for United was exceptional," he said.

"(Given what happens) generally when players go to a new club and have to get used to a new league, team-mates and style of football, the form he is showing now is possibly more typical of a player just going to a club than his first few games, when he burst on to the scene and did so well.

"So I don't think anything is out of the ordinary with his current form."