I think Mourinho has driven United into a cul de sac and doesn't know how to find reverse. United post 2013 feels eerily like Liverpool post 1990. Souness in trying to rebuild Dalglish ageing first 11 replaced established stars with markedly inferior players. I feel this has been the case at OT since 2013. The one player who would walk into every over EPL team is De Gea and he pre-dates 2013. This is after United have spent 670 million since 2013. This is a damning indictment of United's transfers since 2013. Klopp took over in October 2015. He used the remainder of the season to assess what he inherited. He then from 2016 went about rebuilding his first 11. 2016 saw Mane and Wijnaldum come in, 2017 saw Salah and Robertson, 2018 saw Van Dijk, and Alisson. We struggled for goals in 2015/16. He brought in Mane towards remedying this shortfall. He added further firepower in 2017 with Salah. 2016/17 confirmed we were short defensively. He went and got Robertson and Van Dijk in 2018 Jan. He realised that Karius was not going to measure up and brought in Alisson. Sure Alisson slipped up for United's goal but he is still a major upgrade on Mignolet and Karius. Robertson is streets ahead of Moreno. Shaqiri bought in 2018 is already looking a shrewd signing.
Now this is a United forum and I've just gone on and on about Liverpool. Ultimately my point was to show a season on season improvement in the team. To show that Klopp has shown he has a plan to improve the side on a continual basis. From being defensively ropey when he took charge we now have the best defence in the league. He has has to curtail our attacking to do so but it's lead to a better balanced side. Yes he has won nothing but as a team we are markedly better than what he inherited.
Mourinho has won trophies and he deserves credit for that. He has been in the job the same time as Klopp is you go by full seasons. But you are no further down the line as a side than from the day he took the job. I don't see season on season progression in terms of the development of the side he inherited. I don't see where he is going in terms of evolving the first team line up.
2016-2018
Mourinho spent 359m took in 79m
Klopp spent 410m took in 227m
If a new manager came in now it would give him time to assess the squad. The success or failure of any new manager, if/when he is appointed, will depend on recruitment.