9 goals and 10 assists in his first season is a bit better then decent. He was just inconsistent. The year after, dortmund as a whole was horrific. Would you have reservations on signing Reus, Aubamayeng, gundogan or hummels? Over the past 3 years, Mkhitaryan has been as good if not better then all bar Auba. Sure, nobody is a guarantee, but Mkhitaryan with his skill set and work rate is perfect on paper for Mourinho and playing on the right for us. Will compliment Martial well on the other side and give us a lot of much needed pace and creativity out there. Seriously, if people were saying Lingard was doing alright for us, its hard to imagine Mkhitaryan not impressing. Its not like he is replacing ronaldo here.
The player himself said that the €27m fee Dortmund paid for him was a very heavy burden for him and his second season can't simply be explained by the team doing badly, he was terribly shocking at times, there were games when he was almost literally takting the wrong decision at every opportunity (passing too late, losing the ball by dribbling into several opponents, etc), he said himself that he was trying to solve everything by himself which caused massive problems.
I think either he has a good relationship with Mourinho and settles in quickly or he goes down the Kagawa route.