For Manchester United to knock Barcelona out from the Europa League I know they are fit of winning the europa league. Manchester United are with the best squad, players like Rashford and Casemiro always play a good role in every for Manchester United, Rashford is goal scoring machine for Manchester United.
The two players you mentioned were indeed very good players at Manchester United, but there was one player named Bruno Fernandes who sometimes really didn't get along with them at Manchester United because Bruno often made the mistake of not giving feedback to his friends when there was an opportunity so he very often want to score goals alone that sometimes the opportunity becomes very futile. And I think if he can change the way he plays at Manchester United and can mix his game with Rashford and Casemiro, Manchester United will be much stronger than they are now.
Casemiro is a very important player in maintaining the balance of the midfield. Manchester United have been very fortunate to have brought him to Old Trafford this season. He was the first player to win over the ball and would not let the opponent get deeper into the Manchester United defense area. Rashford became a goal-hungry player, he could almost score a goal in every match that allowed Manchester United to achieve positive results.