For example, Philippe Coutinho, who transferred from Liverpool to Barcelona in January can not feature for Barca in the Champions League because he played earlier in the season for the Reds in the competition's group stage.
Europa League matches will kick off at either 1855CET or 2100CET until the round of 16, after which the later time will be used.
The Champions League and Europa League now run in three-year cycles in which no rules are changed and the format remains the same - but next season brings the start of a new cycle.
From next season, clubs reaching the Super Cup and the finals of the Champions League and Europa League will be able to have 12 substitutes on the bench - up from the usual seven.
UEFA have confirmed that it has made some more rule changes regarding its primary club competitions from next season..
The European football governing body said the move will give managers "more flexibility".
In the knockout rounds as a whole, a fourth substitute will be allowed during extra-time, which will not affect the other three substitutions that can be made.
The majority of games will now kick off at 8pm while there will be a number of 5.55pm starts in the group stages as well.
From the 2018-19 season on, teams that have won the UEFA Europa League (including its predecessor the UEFA Cup) three consecutive times or at least five times may wear a multiple-winner badge on the shirt sleeve, as was already the case in the UEFA Champions League.
Kevin Gameiro (L) of Sevilla celebrates with the trophy following the UEFA Europa League Final match between Liverpool and Sevilla at St. Jakob-Park on May 18, 2016 in Basel, Switzerland.
The decision to let clubs register three new players "without any restrictions" after the group stages, however, may be more controversial, as it effectively ends the idea of being "cup-tied".