#4 Being outsmarted by Sevilla over two legs, UEFA Champions League 2017-18
Of all the British sides left in Europe's elite club competition in the round of 16 of the 2017-18 UEFA Champions League, it appeared as though Manchester United's passage into the quarter-finals was the most straightforward of them all. Chelsea had Barcelona, Tottenham were up against Juventus while even their local rivals City had a tricky test against Basel.
Sure, Sevilla would not be pushovers - but it's fair to say that they have regressed somewhat in recent seasons, despite winning three successive Europa League trophies. They've never managed to replicate that success against better sides, so this was expected to be United's litmus test with tougher opposition forthcoming.
They failed, in a big way. Mourinho got his tactics wrong during a frustrating goalless draw in the away leg - opting for Scott McTominay and Nemanja Matic as a defensive-minded midfield partnership, leaving Paul Pogba on the substitutes' bench looked a bold and equally naive decision given his creative qualities.
If not for crucial saves by David de Gea, United could have returned home with a two-goal deficit to overturn. Jose ignored this warning though and fielded a similar defensive-minded eleven, who played with a surprisingly cautious approach. It was cruelly capitalised upon, by second-half substitute Wissam Ben Yedder - scoring two goals in four minutes, leaving Old Trafford stunned as the visitors progressed into the quarter-finals, having taken advantage to devastating effect in Manchester.
He left players including Marcus Rashford, Anthony Martial and the aforementioned Pogba all on the bench in both games - calling upon them late was ultimately futile and Romelu Lukaku's late strike proved nothing but a consolation in the end. Questions have been asked since then, regarding his man-management style, prompting more claims that he's again losing the dressing room as he has done before elsewhere as a manager.