When Rio Ferdinand signed for Manchester United in July 2002, Ruud van Nistelrooy had already been at Old Trafford for a year. The Dutch striker had thrived in his debut campaign under Sir Alex Ferguson, but the Reds ultimately lost out on the Premier League title to Arsenal.
Van Nistelrooy, despite registering a staggering 36 goals in the 2001/02 season, did not win the Golden Boot that campaign - and Sir Alex's role in that has been well documented. However, after adding Ferdinand to their ranks for the following term, United got their revenge on the Gunners and regained their crown.
Having scored 25 goals in the title-winning season, it came as no surprise that Van Nistelrooy was the league's top goalscorer. He finally got his hands on the individual accolade, and much like United did with Arsenal, got his revenge on Thierry Henry after finishing behind him in the charts the previous term.
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Despite winning the 2003 Premier League title - and reaching the League Cup final - the Reds were keen to strengthen. United signed a number of players that summer, including Louis Saha, Kleberson and Tim Howard, but it was the arrival of a tricky Portuguese forward that attracted the most interest.
Cristiano Ronaldo arrived at Old Trafford in that summer's transfer window, and it quickly became clear that Sir Alex had a star in his ranks.
The attacker registered six goals and eight assists in his debut campaign, but it was evident that he was only going to get better at United. The following season, he managed 18 goal contributions. His numbers kept rising, and his impact and influence on the team continued to grow stronger, too.
For Van Nistelrooy, the emergence of Ronaldo didn't impact his joy in front of goal. Moreover, the pair combined for 12 goals during their time together at Old Trafford. Ultimately, though, the Dutch striker left the Reds to sign for Real Madrid just as Ronaldo was hitting his peak at United. They certainly showed that they were capable of playing together, but it just didn't happen as often as fans would have liked.
Given the intense pressure United's players were under during their title battles with Arsenal and Liverpool, it will not come as a surprise that, on occasions, the squad would clash. Most altercations happened in training, and some involved Van Nistelrooy and Ronaldo.
Reds legend Ferdinand admitted training sessions could be tense. On one particular occasion, the former England defender took exception with Van Nistelrooy over a rough challenge on a young Ronaldo. He recalled getting in the Dutchman's face after the striker went in on his teammate's Achilles.
"It was mad at United, things could just switch just like that [clicks fingers] in training," Ferdinand explained on his Vibe With FIVE YouTube show. "I think all training grounds are the same, there’s a lot of testosterone flying about."
Asked if he was ever involved in a fight, Ferdinand said: "It never really got to that. It’s funny, because I’ve got the biggest mouth on the training ground. I used to always be shouting and screaming and I used to love jokes.
"Obviously I got serious in training, with the games and that, especially on a Friday before the game on a Saturday, everyone’s playing, it would be hard games, fast games. One time something happened with Ruud. Ruud van Nistelrooy smashed Cristiano Ronaldo a couple of times, went down his Achilles.
"And then the ball went into Ruud and I smashed Ruud. I said, 'What you doing? Why you kicking him for?' And then Ruud got up and he was [not happy] and all the players just got in the way, so nothing happened.
"But that’s the way it used to be in training. I was thinking, 'Ronnie’s a young kid man, leave him alone, what you doing?' but Ruud was an animal, an animal player."
It would probably be fair to say the pair did not always have the best relationship, but they still enjoyed success together under Sir Alex. They were both part of the side that won the FA Cup in 2004 and the League Cup in 2005.
Speaking in 2019, fellow ex-United star Louis Saha recalled a row the two superstars had during a training session, with the young Portuguese ace in tears in the aftermath. Two years later, Van Nistelrooy admitted he knew he was wrong, but insisted he was happy to apologise at the time.
Discussing his relationship with Ronaldo, he told the FIVE YouTube channel: "The press was going on about that particular incident and I also commented on it, I’m happy to do it again because I know I was wrong in that situation. I immediately walked into the dressing room and sat next to him and apologised for my behaviour when I calmed down.
"That frustration is coming from somewhere and now when you’re 44 you look back and when things happen around you in your private life, it can make you frustrated. It led to a discussion with Cristiano but you have to be big enough to acknowledge that."
The prolific frontman also admitted he would have heated discussions with his teammate, but claimed that was the case for a number of other United stars as well, including club legends Ryan Giggs and Paul Scholes.
He added: "I would communicate with Scholes, Cristiano, [Diego] Forlan, [Jose] Kleberson to try and connect it with them. It was a constant process. Since he became the world’s best player, maybe in history, the discussions go with 'Cristiano and Ruud' and 'Ruud and Cristiano'.
"But no one talks about Ruud and Diego or Ruud and Giggs or Ruud and Scholesy, that’s the situation I think. Even later at Madrid when we played together, when you meet again you know when people are real and that’s the most important thing."
Speaking about Ronaldo two years ago, Ferdinand suggested a different altercation with Van Nistelrooy in training - after failing to provide a simple pass for the striker - prompted an outburst which would eventually inspire the Portuguese ace to ditch the tricks and focus solely on becoming the best player in the world.
The pair would be reunited at Real Madrid for a brief period of time, although they were never actually on the pitch together. After being sidelined with a serious knee injury, Van Nistelrooy came on for Ronaldo in his first La Liga match since completing his recovery in 2009, but he picked up a thigh injury and was ruled out of action for another six weeks.
He departed the Santiago Bernabeu Stadium the following January, with Ronaldo going on to win countless trophies and break several records during his time with Los Blancos.
While his return to Old Trafford ended on a sour note, Ronaldo - much like Van Nistelrooy - enjoyed a hugely successful career in a red shirt. He scored 145 goals for United, winning eight major honours with the club. Meanwhile, Van Nistelrooy was on the scoresheet on 150 occasions, winning three pieces of silverware.
Although they came to blow on occasions, there was - and still is - a clear level of respect between the two players. Ferdinand has Ronaldo's back in training, but Van Nistelrooy's tough love arguably played some part in his development as a youngster.
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