Cast your mind back to the summer of 2018.
England reached the World Cup semi-final, Gareth Southgate thrilled the nation with his famous song and waistcoat, and of course, who can forget Harry Maguire riding an inflatable unicorn?
OK, that didn't happen - but what if did?
Where would United be now? And where would they have finished in the Premier League this season?
We've re-written history using Football Manager 2019 - drawing upon a scenario several at Old Trafford would like to see - to find out exactly what would have happened if they made that change last summer.
The Argentine would join United on 1st July 2018, and he wasted no time splashing the cash.
On July 2, the club completed the signing of Torino striker Andrea Belotti for a transfer fee that could rise up to £34million. Would he become Pochettino's new Harry Kane?
At the start of August, with just a few days left in the summer transfer window, the club completed their second, and final, transfer of the window. Spanish defensive midfielder Ignacio Camacho arrived from Wolfsburg, for a transfer fee that could rise up to £26.5million, as the new boss added a bit of steel to his midfield.
Then, comes the start of the 2018/19 Premier League season. Have a guess who United play in Pochettino's very first league game in charge... yes, Tottenham Hotspur at Old Trafford.
Goals from Alexis Sanchez and a debut double from new signing Bellotti seal the three points against his former team, in what is a great start to the season.
This opening day win spurred United on and they embarked on a phenomenal run of form, with Pochettino's first league defeat as manager coming against fierce rivals Liverpool in late November. Goals from Roberto Firmino and Mohamed Salah silenced the Old Trafford crowd.
Rather incredibly, they wouldn't lose another league game all season.
Colombian midfielder Wilmar Barrios and right-back Rick Karsdorp arrived from Boca Juniors and Roma respectively, as United strengthened in January, to give them a boost in their race for the Premier League title.
Man United would go on to win their first league title since the 2012–13 season, as Pochettino's first campaign as manager proves hugely successful.
They finished with 91 points, of which there were 27 wins, 10 draws, 65 goals scored and just one single defeat. They also registered 19 clean sheets, more than any other team, and only conceded 25 goals, less than any other team.
Italian hitman Belotti finished as the club's top scorer in the league with 18 goals, but the Golden Boot went to Arsenal's Pierre-Emerick Aubameyang with 23 goals.
Despite Pochettino's brilliant Premier League triumph, the Argentine would have to face disappointment in the cup competitions.
They lost on penalties to Wolves in the Carabao Cup third round and also lost 2-1 to local rivals Man City in the FA Cup sixth round.
But the cruelest blow would come in the Champions League.
United beat Spartak Moscow, Atletico Madrid and Real Madrid in the knockout rounds, before losing to Liverpool on penalties in the final. Paul Pogba and Jesse Lingard both missing penalties in the shootout.
However, all in all, it was a very successful 2018/19 campaign with the Argentine in charge.
United fans might be happy with Ole Gunnar Solskjaer right now, but this shows what might have been - had they replaced Jose Mourinho with Mauricio Pochettino this summer.