UEFA Europa League has never been predictable with top teams because mostly they never like to play in this league mostly while these are having exit from the Champions League these are staying for the one or two rounds and then have exit which is common business for them but in this season we have few interesting matches which are creating good momentum for the first knock-out round because Manchester United is having good time and recently their performance level is also increased which is clearly indicating they are going for win and have fair chance to win this title but on other hand Barcelona is also improving their quality even their main target is La Liga, but they can do better in this league as well which is surely amazing and interesting for the first knock-out round which will played after four weeks who could be winner this is surely not easy to give exact reply.
EL is often unpredictable, you are right, there are various reasons for this, lack of motivation, or lower priorities over other tournaments. Therefore, it will be very difficult to predict who can be stronger Barcelona or Manchester United until we see the first game. They are close to their optimal shape, so if each of the teams is determined to win, we will see an amazing confrontation between the two European giants at such an early stage.