Cristiano Ronaldo is threatening to become a major problem for Manchester United with the veteran forward attempting to argue with Erik ten Hag’s tactics, according to reports.The five-time Ballon d’Or winner has made it clear that he wants to leave Old Trafford this summer so he can continue playing in the Champions League, though he has received significantly less interest than he was expecting while United do not want to sell their top scorer from last season.Ronaldo was given permission to miss the club’s pre-season tour of Thailand and Australia for ‘family reasons’ and was only fit enough for a place on the bench in their season opener against Brighton.He returned to the starting XI in Saturday’s 4-0 mauling by Brentford but cut a dejected and frustrated figure throughout, while he refused assistant coach Steve McClaren’s request to acknowledge United’s away fans after the game.There have been reports that Ronaldo’s sulking is starting to affect the rest of the team, with many players feeling it would be better if the Portuguese superstar just left the club.Now The Athletic claim the wantaway 37-year-old’s mood and behaviour is becoming so openly rebellious that he has tried to argue against Ten Hag’s high-pressing tactics in training.The Dutch manager’s style of play hinges on having a dynamic and hard-working attack but pressing from the front and closing down is not something Ronaldo is particularly known for or enjoys – even when he was in his prime.Indeed, he was criticised for his lack of work rate at times last season despite leading the club’s scoring charts and looked a poor fit for interim manager Ralf Rangnick’s system – who also tried to engage the opposition high up the pitch.But Ronaldo has been seen