Jose Mourinho vs Pep Guardiola - a battle between two gladiator managers with a colourful history. The early stages saw both clubs go blow-for-blow as United won six of their first seven and City also won six of their first seven.
A title race looked certain.
But then United looked mortal while City blew away everyone with their scintillating football and star power. The gap between the two teams now stands at 17 points.
For Mourinho, this season has been a failure, an early Champions League exit coupled with a failed title bid and disappointing League Cup exit at the hands of Bristol City show that.
However, there is one final chance for Mourinho to save the season - by beating his former club Chelsea in the FA Cup final.
Ever since Sir Alex Ferguson left Manchester United in 2013, the club have struggled with their identity.
David Moyes was an unmitigated disaster, Louis Van Gaal oversaw an underwhelming tenure and Mourinho has yet to turn Manchester United into the title-winning monster of yesteryear.
This is why the FA Cup final is so important. In 1990 Fergie picked up his first of 38 trophies with a crucial FA Cup trophy.
Mourinho and his team picked up three trophies last season and an FA Cup this season would just start to create that winning mentality that Manchester United craves.
Building block for next season
Importantly the FA Cup presents United with the chance to enter next season with momentum. This year they fell woefully short of Manchester City who swept everyone aside, next year the team needs to be able to match them.
These big games can tell Mourinho a lot about his team and give him an indication of which players will be fit for the challenge of next season.
More important than that though, is the feeling of triumph to end the season.
If United can win the FA Cup, then Mourinho will have picked up four trophies in just two seasons - impressive for a manager who has taken a lot of stick this season.
Champions League qualification, a second place finish, and an FA Cup trophy will go a long way to silence the doubters at Old Trafford.
Defeat is unthinkable
Jose Mourinho would be lying if he said that the final did not mean a little bit more because he was facing off against his former club Chelsea. Defeat would be a bitter blow and might save his successor at the Blues, Antonio Conte, job for next season.
If Manchester United are defeated, then their fans will rightly be concerned that they are not progressing at a quick enough pace.
For all the money Manchester United spent this summer, they will have failed to register a trophy while falling woefully short in the Champions League and Premier League.
Mourinho, therefore, will know that defeat is not an option for his team.
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