The Champions League showpiece will be held at the Wanda Metropolitano, home to Atletico Madrid, on June 1, with the first Nations League semifinal taking place four days later.
Hosts Portugal are due to appear in that first semifinal on June 5, which is set to be played at the home of FC Porto, with the second semifinal being held 24 hours later in Guimaraes on June 6.
It means the hosts, as well as fellow Nations League finalists England, Netherlands and Switzerland, could be denied key players in the lead up to those semifinal ties -- if they are even called up at all.
England have the highest chance of being adversely affected, with Manchester City (four players) and Liverpool (four players) backed to go far in the Champions League.
Netherlands also have two Liverpool players, plus Barcelona reserve goalkeeper Jasper Cillessen.
Portugal could be without Juventus' Cristiano Ronaldo, who did not play in the Nations League group stage, and Joao Cancelo, plus they also have players at Atletico Madrid, Barcleona, Bayern Munich, Borussia Dortmund, Inter Milan, Manchester City and Napoli.
Switzerland would appear to have the least chance of being affected.
UEFA decided to play the semifinals on June 5 and 6 to ensure they are standalone games, with all other European countries playing in Euro 2020 qualifiers on June 7 and 8. The Nations League third-place playoff and final follow on June 9.
FIFA rules state that players must be released for a major finals tournament 14 days before it begins, but the Nations League is being played on a designated international week in the football calendar and UEFA would not want to devalue the Champions League by enforcing call-ups.
Here, we see how the UEFA Nations League finalists could be affected if their players are in the Champions League final.
Borussia Dortmund: Jadon Sancho
Liverpool: Joe Gomez, Trent Alexander-Arnold, Jordan Henderson, Adam Lallana
Manchester City: Kyle Walker, John Stones, Fabian Delph, Raheem Sterling
Manchester United: Luke Shaw, Jesse Lingard, Marcus Rashford, Phil Jones
Tottenham: Eric Dier, Dele Alli, Harry Winks, Harry Kane, Kieran Trippier, Danny Rose
Ajax: Matthijs de Ligt, Daley Blind, Frenkie de Jong, Donny van de Beek
Barcelona: Jasper Cillessen
Inter Milan: Stefan de Vrij
Liverpool: Virgil van Dijk, Georginio Wijnaldum
Lyon: Kenny Tete, Memphis Depay
Roma: Justin Kluivert
Atletico Madrid: Gelson Martins
Barcelona: Nelson Semedo
Bayern Munich: Renato Sanches
Benfica: Ruben Dias, Pizzi, Rafa Silva, Gedson Fernandes
Borussia Dortmund: Raphael Guerreiro
Inter Milan: Joao Mario
Juventus: Joao Cancelo, Cristiano Ronaldo
Lyon: Anthony Lopes
Manchester City: Bernardo Silva
Napoli: Mario Rui
Porto: Danilo Pereira, Sergio Oliveira
Valencia: Goncalo Guedes
Benfica: Haris Seferovic
Borussia Dortmund: Roman Burki, Marwin Hitz, Manuel Akanji
Galatasaray: Eren Derdiyok
Hoffenheim: Steven Zuber
Liverpool: Xherdan Shaqiri
Schalke: Breel Embolo