Mkhitaryan, Alexis, Pogba(*1st time)...Forget Mourinho for a sec, Utd's problem is partly their transfers and scouting. They've bought Diogo Dalot this year so we'll see what happens there, but before that I can only think of Martial and Shaw in recent times as hot young prospects they've brought in. Both unmitigated failures so far, for different reasons.
They need to rediscover the knack they used to have for bringing in good young players.
Oh wait there was also Depay and Zaha. Also unmitigated failures.