Manchester United is aiming at winning the trophy so that they can automatically get a ticket to UCL, and to achieve this goal, United have been performing pretty good in this competition, despite the challenges that they are having in EPL. Tottenham is also in the same shoes with United but United current performance looks more promising to win the league than Tottenham, because Amorin is still building the team, and we might see a sudden change in their performance.
I don't think we should expect a major transformation from United until transfers are made and until Amorim fully prepares the team for the season. In this sense, I think Tottenham are doing better. As for United's success in the Europa League compared to the Premier League, it's simply because of the difference in the level of the teams. Plus, even taking this difference into account, United have looked frankly weak in a large number of games in the Europa League.