Some teams seem to cope better than other when juggling the Champions League/Europa League with domestic duties. Obviously squad depth is a factor here. Some interesting analyses on the domestic performance both before and after midweek European games: http://www.thestatszone.com/articles/how-much-does-the-champions-league-affect-domestic-form http://www.thestatszone.com/articles/does-europa-league-affect-domestic-form Sort of leads to the conclusion that it's a myth!
I think Mourinho's squad selection says everything although they have the squad to rotate and stay strong. It may be more of an issue for Southampton who don't have quality depth in the squad to juggle.