Clubs competing in the Champions League, Europa League and Europa Conference League will be clear to replace long-term injured players with a temporary substitute up until the knockout phase of each competition.
It means some of the big names not registered by their clubs for the group phase could yet see European action.
In England, Spurs left out £30m France under-21 ace Mathys Tel, Liverpool omitted Italian Federico Chiesa, while Arsenal had no place for Gabriel Jesus and on-loan Facundo Buononotte missed out for Chelsea.
Uefa said: “The reasoning for the adaptation is to ensure that squad lists are not unfairly reduced and players are safeguarded from additional workload pressure.”
The sudden change is expected to see Chelsea name Buonanotte as a replacement for Liam Delap, who will be missing until December due to a hamstring injury.