Here, football.co.uk assesses five of the most pressing issues.
1
CHARM DE GEA
It is hard to over-estimate the importance of goalkeeper David de Gea to United's short-term prospects.
He is not only the club's best player - and possibly their only world-class one - but also a beacon to potential future signings
Holding on to him may be tough without Champions League football, but would be the best piece of business United could do this summer.
Mourinho can command fierce commitment among those he values most, though, and if he can make De Gea believe in his cult of personality it could be the trump card the Red Devils need.
2
UNIFY THE CLUB BEHIND HIM
Mourinho was somewhat unsubtle in pitching to be Sir Alex Ferguson's replacement three years ago but was overlooked amid suggestions that important voices behind the scenes, including Sir Bobby Charlton, did not see him as a 'United man'.
After failed experiments with David Moyes and Louis van Gaal, Mourinho looks to have his wish.
Instead of antagonising those who argued against his appointment, or crowing about the club's recent struggles, he must work over-time to win over his doubters.
It has been too long since there was a true consensus and the Portuguese may have to play against type to build one.
3
OVERHAUL THE TRANSFER POLICY
Since Ferguson and former chief executive David Gill departed in 2013, United's transfer business has too often veered between confused, rushed, and incompetent.
They have allowed themselves to become a bargaining chip for agents, missed out primary targets and been overcharged for others.
The investment in the past three seasons has been sizeable but the returns modest.
Mourinho is forceful enough to take the reins from Gill's successor Ed Woodward, or at least push him to be more effective
He is not used to seeing his shopping lists unfulfilled and will not want to start now.
4
REVIEW HIS YOUTH POLICY
Arguably the greatest success of Van Gaal's tenure was the number of academy graduates he blooded in the senior side, whether by design or circumstance.
Marcus Rashford and Jesse Lingard look ready to play big parts in the future, while Cameron Borthwick-Jackson and Timothy Fosu-Mensah have also showed promise.
At Chelsea the most sustained criticism of the 'special one' was his disinclination to promote players from the Blues' hugely successful Under-21 team.
United fans love to see locally developed stars emerge and would not be overjoyed to see Mourinho's stunt their progress.
5
IS HE ANTI-MATA?
United's FA Cup win would not have been possible were it not for Juan Mata's equalising goal in the 81st minute.
The Spaniard has made plenty of telling contributions in his time at Old Trafford but the feeling remains that he has never been treated as a central pillar of the side.
He would, therefore, be forgiven for thinking the arrival of the man who sold him to United in the first place was not great news.
Mourinho certainly failed to warm to Mata at Chelsea but may need to revise his opinion.
Source : PA
Source: PA