The Europa League has its own level of difficulty because it is play by teams from several leagues, not the same as what they face in the premier league.
If they feel the chances of winning the EPL title will be tough then the same will happen in the Europa League as Manchester United will be competing with the like of Freiburg, Barcelona, Feyenoord, Sociedad and Arsenal.
The Europa league is not as difficult as the Champions league. In the Champions League their are lots of tough teams to face while in the Europa League it consists of many small teams and few big teams. In the europa league big teams are always at the top of performance.