Manchester United needs to focus more on UEL than EPL because they can easily succeed in UEL. EPL is very tough for Amorim with the set of players in the team and there is no need putting too much attention there. What Amorim needs to do is to make sure United stays in a good position on EPL table and qualify automatically with UEL to UCL next season. He can achieve it, if him and his men are determined and improve their performance constantly.
I think the first phase match in the Europa League looks easier for Manchester united to beat their opponents but in the round of 16 and the next stage the competition will be stronger and it will be a tough task for them. But it is true as you said that they have to focus as best they can in the Europa League because it will bring them to the Champions League if They can become champions.
In addition, in the Premier League, manchester united performance can be improved slowly and achieve a better ranking, but to reach the top 4 of course they will be overwhelmed to achieve it. So one option is to focus their energy as best they can in the europa League match, the next match manchester united will face FCSB.