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Manchester United agreed to sign Ruud van Nistelrooy from PSV Eindhoven for a fee of £18.5million in April 2000.

Van Nistelrooy had just contributed 32 goals and eight assists in a season for PSV and Sir Alex Ferguson was desperate to bring the Dutchman to Old Trafford. Van Nistelrooy was 23 years old and hungry to make his mark in the Premier League.

The transfer made sense for all parties involved, but the deal collapsed in late April with concerns about Van Nistelrooy's fitness. The player had sustained a knee injury eight weeks before his medical at United and he was deemed unsuitable for transfer.

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In a statement the club said: "Manchester United is very disappointed that the transfer of Ruud van Nistelrooy from PSV Eindhoven cannot proceed at this present time. The discussions that have been held between all the parties failed to reach agreement as to the extent of these tests and as a result of this difference in opinion, we cannot proceed at this present time."

Just days later, Van Nistelrooy returned to PSV and ruptured his cruciate ligament in the knee which was the source of concern. United reiterated it was their desire to complete the deal when Van Nistelrooy was fit and they stayed true to their word.

Although Van Nistelrooy would have to wait another year for his big move, he finally signed for United on 23 April 2001. The Van Nistelrooy episode demonstrated why footballers' medicals are so important and why they are non-negotiable in transfers.

The majority of players will pass their medical as a formality, but it's not that simple for others, so what exactly happens in a medical? The Manchester Evening News visited private