In a usual La Liga matchday squad, a team is only allowed to name three non-European Union players, and many had thought the rule would carry over into the Supercopa de Espana.
Messi, who will don the captain's armband for the first time after the departure of Andrés Iniesta to Vissel Kobe during the summer, will also overtake the midfield maestro as the player with the most titles for Barcelona.
Sunday's Spanish Super Cup final may not go ahead after Sevilla threatened to pull out of the match if Barcelona field more than three non-EU players. In their last statement for the 2018-2019 season, only three were due to be allowed, without exception.
But Barcelona are keen to avoid any clashes with Sevilla and have erred on the side of caution by leaving Malcom out of the squad and have made a decision to start Arthur, while Coutinho and Vidal have been named as substitutes.
Barcelona aren't expected to stay in Morocco and will instead fly back to Spain immediately following the conclusion of the match. Barca have taken four non-EU players to Morocco, including defender Marlon.
Philippe Coutinho has recently acquired a Portuguese passport through his wife, which means he is no longer considered a non-EU player.
Despite the adjustment potentially giving his side an edge, Barca boss Valverde is also far from content. "With all the trouble we have had this week with non-EU squad members, they could have warned us before".