#4 Negligible Player Development
The fact that Manchester United went backwards with Mourinho at the helm is evident from the fact that good players seemed to lose their touch and abilities with time under the Portuguese. Marcus Rashford is perhaps the biggest example of this, as the teenager has looked like a shadow of himself recently. Even older, established players failed to develop themselves due to Mourinho's tactics. Players like Pogba and Lukaku started this season well, but seem alarmingly short of confidence at present.
On the other hand, Fellaini has continued to feature in Mourinho's plans, limiting the chances for promising young talents like Andreas Pereira. Bailly and Lindelof, players identified by Mourinho himself, were suddenly not good enough for the Portuguese anymore. Bailly's absence this season has been shocking, especially since he was rock solid last season. No academy graduates moved up the ranks to make their mark in Mourinho's team and this was also a big problem at Old Trafford.
What made matters worse were the reports of Player-Manager tussles. There were rumours of building differences within the team and also the management's reluctance to indulge in Mourinho's choices in the transfer market. Ultimately, there were too many problems associated with the Portuguese and United were correct to seek a fresh start by breaking free from Mourinho.