1. The "Third season syndrome"
In every second season at every club he's been at, Mourinho has never failed to win the league but into his third and it all starts to unravel.
He only spent two full campaigns at Porto and Inter, but his third terms at Chelsea (both in his first and second spell) and at Real were deeply problematic and ultimately led to his departure as manager.
Despite his suggestions to the contrary, there's clear evidence that Mourinho generally leaves clubs under a cloud after two and a bit seasons, and this recurring pattern is a concerning blot on his copybook that will be hard to ignore for the club's powers that be.
2. Lack of youth development
Interwoven in United's history, the development and nurturing of young homegrown gems is as much as part as the club's history as trophies and attacking flair.
Sir Alex Ferguson did so with his Class of 92, David Moyes takes the credit for Adnan Januzaj's talent and Louis van Gaal has done his best to keep the tradition going. Yet one of the major drawbacks of his career is that Mourinho has not put enough focus on the academy and youth setups of his various clubs - ignoring Academy talent and preferring to heavily recruit from abroad.
3. He's controversial and confrontational
Arsene Wenger, Carlo Ancelotti, Rafa Benitez (even his wife) and - yes - Pep Guardiola.
Just a few of the high profile spats involving Mourinho and his fellow managers down the years and it's his history of feuds, tantrums and misconduct that is said to have played a part in why he was initially overlooked to succeed Sir Alex in 2013.
Poking Tito Vilanova in the eye, his frequent outbursts, outspoken manner and individual agenda is not behaviour befitting of a United manager and borders on embarrassing but will the club turn a blind eye or will his histrionics forever be an insurmountable Old Trafford obstacle.
4. Short term success but no long term legacy
Mourinho is not the answer for United - not in the long term. We've talked about his history of short term success, but he's never stayed longer than three seasons at any team and for a club that prides itself on longevity that has to be a potential drawback.
That said, both of United's last two managers have been short-term appointments as seems the norm nowadays, and United could be the club that Jose decides to settle down, build a legacy and finish his career with but history would suggest otherwise.
The club are still thought to be keen on the idea of Ryan Giggs succeeding Van Gaal, which is another issue entirely but would surely be the man to take United into the next decade and beyond.
5. Attack, Attack, Attack
Louis van Gaal has come under pressure for his perceived style of play and negative tactics throughout his time at United- but Mourinho is also famed for his pragmatic and "park the bus" approach.
His prefence of substance over style is unquestionably effective but the job of a United manager is to marry the two together.
In his second season at Real Madrid, Mourinho's team outscored Pep Guardiola's Bayern and his Chelsea champions hammered six past Everton and Arsenal during his second tenure, but his reputation as a win-at-all costs, grind it out tactician have followed him throughout his career.
United could be risking a step away from their identity by appointing Mourinho - but with Guardiola across town, it might be a risk worth taking.