Cristiano Ronaldo is one of the greatest players to have ever played the game.

He may now be 37 but the Portuguese superstar is still going strong at the very top of the sport.

But Ronaldo has worked extremely hard to get to achieve everything he's achieved in football.

You don't win five Ballons d'Or and continue playing Premier League football at the age of 37 without a lot of hard work and dedication.

After arriving at Manchester United for £12.24 million in 2003 as an 18-year-old, many doubted the skinny winger was capable of playing in England. In his first few appearances, he appeared more interested in producing stepovers and flicks than scoring goals.

But it wasn't long until he was tearing up the Premier League.

While he deserves a lot of credit for the transformation, it was clear he was at the right place with the right people around him.

Sir Alex Ferguson was like a father figure to him for many years.

Meanwhile, figures like Roy Keane, Gary Neville, Rio Ferdinand and Paul Scholes ensured Ronaldo did everything correctly both on and off the pitch.

And a short clip showing from training shortly after Ronaldo joined the club shows exactly that.

In the clip, you can hear Man Utd icon Keane having a go at Ronaldo for failing to track back during a training game by shouting 'C'mon Cristiano. Cristiano C'mon' as the opposition team score.

Cristiano Ronaldo & Roy Keane in Man Utd training

It may just be a short snippet but it shows the standards at Man Utd at the time and how Keane made sure everyone was held to those high standards - no matter who they were.

Samuel Luckhurst, Chief Manchester United writer for the Manchester Evening News, posted the clip earlier this week with the caption: "Great footage from the encyclopaedic @utd_clips of Keane berating an 18-year-old Ronaldo for failing to track back in training. Ronaldo was inside United’s own area mucking in as they held onto leads last season."

Check it out:

VIDEO: Roy Keane shouts at a young Cristiano Ronaldo

There has been speculation this summer of Ronaldo leaving Old Trafford but, last month, Keane insisted there was no logic in that.

"It [Ronaldo's signing] was a short-term fix, for the fans, for the share price. But he has still scored the goals," Keane said on Monday Night Football.

"Manchester United have bigger problems than Cristiano Ronaldo.

"I just think with the goals that he has got, his performances, warrants staying at the club. But Manchester United have to get players around him, they have to get other strikers in, there has to be competition for places.

"If the leading goalscorer, the man you are depending on, is 37 years of age then that is certainly not good but I would certainly be holding onto Ronaldo.

"There might be a conversation that needs to be had about the way you are going to play or that you are not going to be playing week in and week out. But why would you get rid of a player who scores that many goals? I don't see the logic in it."