Per The Sun, UEFA's new regulations on multi-club ownership means that clubs owned by the same party cannot compete in the same European competition. New United co-owner Ratcliffe also owns Ligue 1 club Nice, and at the time of writing, both clubs are set to qualify for the Europa League, even if the Premier League gains an extra Champions League place.
Were they each to qualify for the same competition, precedence would be given to the team that finished highest in their domestic league, which is currently Nice. United may well need to win the FA Cup to ensure they are granted priority.
However, there remains a situation in which both teams qualify for the Europa Conference League, and Nice do so via a higher Ligue 1 finish than United's Premier League ranking, and that would leave the Red Devils without any European football.