Chelsea will have to leave one of their big name January arrivals out of their Champions League squad in a headache for manager Graham Potter.
The Blues are set to face Borussia Dortmund in the last-16 of Europe's top competition, with the first leg in Germany on February 15 and the return at home on March 7.
Owner Todd Boehly has splashed the cash during January, with Enzo Fernandez becoming their eighth signing of the transfer window in a British record £107million deal on deadline day.
His signing took Chelsea's January spending to more than £323m.
Other arrivals at Stamford Bridge include on-loan Joao Felix, £88m Mykhailo Mudryk and £35m Benoit Badiashile but UEFA rules stipulate only three new players can be registered for the knockout stages.
Article 46.01 of the Champions League regulations states: 'As of the round of 16, a club may register a maximum of three new eligible players for the remaining matches in the current competition.
'Such registration must be completed by 2 February 2023 (24.00CET) at the latest. This deadline cannot be extended.'
It means Chelsea must decide which three of their eight January signing to add to their squad before Thursday night.
There is some wriggle room because Chelsea only named 24 players of a permitted 25 on their List A for the group stage, leaving open an extra space.
It means Chelsea can only add one of Fernandez, Felix, Mudryk and Badiashile initially before they have to remove existing senior players from the list.
However, because of the 'three new players' rule, one of that quartet is going to be left off anyway.
Article 46.03 of the rules says: 'If the registration of such new players causes the number of players on List A to exceed 25, the club must remove the necessary number of currently registered players to reduce the squad to 25 players again.'
It would also appear other January arrivals such as Malo Gusto, Noni Madueke, Andrey Santos and David Datro Fofana will miss out on Champions League football this season.
Another complication is that Chelsea must have eight 'locally trained players' in the 25.
UEFA divide this into 'club-trained players' - those who between the age of 15 and 21 were registered with Chelsea for a period, not necessarily continuous, of three seasons or of 36 months.
Within the eight, they can have a maximum of four 'association-trained players' who between that age range were registered for three seasons or 36 months with another English club.
Potter could omit senior players currently injured to make way for some of the new boys, such as Wesley Fofana or Christian Pulisic. Loanee Denis Zakaria could also be chopped.
But they cannot remove the injured Armando Broja because he was on List B, for under-21 players who have been at the club for a certain amount of time.
Chelsea won't face similar problems registering the players for the Premier League, whose rules are less rigid than UEFA's.
The Blues took their signing of Fernandez from Benfica right up to the wire on Tuesday night, breaking the British transfer record to seal a £107m deal.
The Argentina World Cup winner has signed an eight-and-a-half year contract until June 2031.