Manchester United is currently ranked sixth in the league, while Arsenal, Manchester United and Everton, ranked fifth to seventh, won the Europa League next year.
If Manchester United win this year's Europa League (beat Ajax), Manchester United will get the Champions League seat directly;
If the final fails, Manchester United will continue to play in the Europa League next year.
The result of Manchester United's final will affect whether one more team in the Premier League can play in the Champions League, but it has no effect on Arsenal, which ranks fifth.