The summer transfer window slammed shut last Thursday after months of rumours, speculation and deals. 

The biggest move was perhaps Neymar's record breaking £198 million switch from Barcelona to Paris Saint-Germain. 

It was so big in fact, that the French club are now being investigated by UEFA over fears they've broken Financial Fair Play rules.

But they weren't the only club buying big over the summer.  

Premier League clubs spent a record £1.4 billion over the last few months, breaking last years record by some distance.  

Both Chelsea and Manchester City spent over £150 million, with the Blues securing the likes of Alvaro Morata, Tiemoue Bakayoko and Danny Drinkwater.   

Pep Guardiola's City concentrated on strengthening the defence, bringing in Kyle Walker, Benjamin Mendy, Bernardo Silva and goalkeeper Ederson. 

Their rivals Manchester United also spent big. Jose Mourinho brought in Romelu Lukaku, Victor Lindelof and Nemanja Matic.  

But there was one huge transfer that would have topped all those signings.  

Early on in the summer, Cristiano Ronaldo was linked with a return to Old Trafford. Allegations of tax fraud unsettled the forward so much he was apparently ready to leave Spain altogether.

However, nothing ever materialised as the player himself remained silent, so the rumour disappeared.

But now reports suggest that Ronaldo's move to United could have been possible, had Kylian Mbappe not scuppered that chance.

According to the Sunday Times, as per Goal, Madrid would have been willing to let the 32-year-old leave, if they could have secured the transfer of Kylian Mbappe.

However, the 18-year-old Frenchman and his father reportedly didn't want to take the burden of being Ronaldo's direct successor, so preferred a move to PSG instead.

It seems that had Mbappe been open to the challenge of replacing the world's best player, this summer could have been very different for PSG, Madrid and United.