The obvious result would be many teams will lose invaluable players and some matches won't even be able to be held in the UK — just the latest example of the Brexit effect on the culture, economy and politics of the UK.
In a US media interview, Slovenian UEFA President Aleksander Ceferin said if Brexit happens "everything changes."
He warned if the UK's scrapping of free movement rules from the EU makes it harder for players and fans to enter the country, a severe rethink will have to be had about where future games and competitions are held.
While insisting his organization remains committed to holding the 2020 European Football Championship in London, he warned UEFA would be in contact with the British government and the English Football Association in the coming months and years over the issue.
Several football players have been refused entry to the UK in recent years — for instance, in November 2016 Ivory Coast player Serge Aurier was unable to compete in a match against London club Arsenal after the Home Office revoked his visa.
Aurier had been found guilty of assaulting a police officer, and was appealing a two month prison sentence at the time, so was free to continue playing football in his home country The UK authority had initially agreed to issue a visa but did a volte-face, a decision UEFA dubbed "regrettable".
Ceferin said this phenomenon will worsen when Brexit happens, "especially if the reasons were as formal."
"We could have a serious problem, although I don't object to authorities withholding visas from severe criminals," he explained.
Among those who could be affected are two of football's biggest attractions, Argentinian and Brazilian players Lionel Messi and Neymar da Silva Santos Junior, both players for Barcelona.
Messi, widely regarded as the best footballer in the world and one of the greatest to ever play the game, is currently appealing a 21-month prison term that a Spanish court handed down to him after being found guilty of tax fraud.
Last month it was announced that Neymar will stand trial on corruption charges after losing an appeal. The case revolves around the Brazilian's high-profile transfer to Barcelona in 2013.
"Both have procedures going on. This year, the Champions League final is in Cardiff. Imagine if they did not let them in. That is a big thing for us," Ceferin warned.
Ceferin added that any country wishing to host the FIFA World Cup, the world's most watched sporting event alongside the Olympic Games, would be disadvantaged by restrictions preventing players, fans or journalists from attending.
The UK is not alone in potentially losing its ability to host footballers. Ceferin indicated any country bidding to hold the World Cup, the most watched sporting event in the world alongside the Olympic Games, would be disadvantaged by restrictions potentially preventing players, fans or journalists from attending. The US is a favorite to host the 2026 tournament, either alone or in a joint North American bid — in direct reference to Donald Trump's alleged immigration restrictions, Ceferin, Vice President of FIFA, said he was sure they would "not help" the US get the World Cup.