There has been a lot of talk about why Cristiano Ronaldo has risen to a whole different level since his departure from United in 2009, and why Wayne Rooney's development has stalled somewhat.
Considering both players were arguably on par with one another at United, it's staggering to see how much better Ronaldo has gotten compared to Rooney.
Liverpool and Germany legend legend Dietmar Hamann believes it is simply Ronaldo's work ethic that separates him from United's new captain.
He told Perform: "You have to live like a top player. If you want to be the best, you have to live like the best.
"And if you like to party on the weekends and drink beer or a glass of wine too much that could be negative for your performance and I think that's the big difference between Rooney and Cristiano Ronaldo.
"Rooney always knocked on the door to be world class but never did the final step. And Ronaldo overtook him because he's obsessed."
The 28-year-old Englishman has been criticised in the past for his attitude to fitness, with pictures of him smoking and drinking during the off-season sparking such criticism. However in recent years Rooney's fitness has come on leaps and bounds, and you can't really question his work ethic on the field, where he is quite often one of the players who works the hardest for his team.
Hamann goes on to add that it's the Real Madrid man's dedication to practice and training that has put him way ahead of Rooney.
"He's a real hard worker. The way he shoots his free kicks is the result of years of training. That doesn't come overnight.
"As I said, often the really talented players tend to do a little bit less because they can get away with it and the others are not as talented as they are.
"But if you have the talent, the morale and if work really hard just then you'll become a Ronaldo or a Messi."