Now, this once-fantasy is becoming a plausible reality.
Per The Guardian's Jamie Jackson and Dominic Fifield, the two clubs have agreed on a provisional fee of £40 million, while ESPNFC's Miguel Delaney reports that they hope to complete the deal by the weekend.
Mata was considered by many to be Chelsea's best player last season, as he racked up 20 goals and 28 assists in all competitions and was named to the PFA Team of the Year.
This season, however, Mata has not had the same influence, as new manager Jose Mourinho has preferred Brazilian starlet Oscar in the spot behind the striker. The Spanish international has just one goal and two assists in all competitions, and he has only started 11 of Chelsea's 21 EPL matches.
His last start came on New Year's Day vs. Southampton, and he has since been an unused substitute three times.
Mata's world class ability, as well as the lack of appreciation of him at Chelsea, have put the Spaniard on Manchester United's radar, as they try to make a second-half push this season.
What he would bring to United
Though Mata is not the box-to-box midfielder that United fans have been calling for, he is undoubtedly a world-class player and an attack of him, Wayne Rooney and Robin van Persie would be feared across the league.
While some would claim that United already have a creative midfielder in Rooney, United need depth at every position to return to being a truly great team. Their lack of depth has been on full display as of late, with the absences of van Persie and Rooney leading to losses against Sunderland and Chelsea.
In addition, the signing of Mata would allow for Rooney to play in his natural position of striker, as he wouldn't have to play the No. 10 to hide a struggling Shinji Kagawa.
Much of United's struggles this year has come from their difficulty to play through the middle of the field. Per WhoScored, United have attacked through the center of the pitch at a league-lowest rate of 24 percent, while attacking through the right at a league-high 44 percent.
Conversely, they cross the ball a league-high 27 times per game, whilst only playing two through balls per game.
Ranking fifth in pass success percentage and eighth in average possession, they don't seem to have too much trouble holding onto the ball, but rather obtaining it in more favorable positions.
Despite crossing the most times per game this season, United have only taken 65 percent of their shots from the middle of the field—12th in the league. Meanwhile, they've chosen to take 20 percent of their shots from the right—second most in the league.
What these stats show is that United's crossing has been poor and mostly fruitless, while they have also not been able to find any creativity or end product in the middle of the field.
That's where Mata steps in.
The Spanish midfielder is a master of creating for his teammates, as he trailed only David Silva and Leighton Baines with 2.7 key passes per game last season, per WhoScored. Mata also had 1.3 successful crosses and 2.8 accurate long balls per match, showing that he strived to do more than just keep possession.
Under Mata's leadership, last year's Chelsea team played efficiently in the middle of the field, attacking there 31 percent of the time and taking 69 percent of their shots there.
Chelsea, carried by the world-class play of Mata, trailed only Manchester United with 75 goals on the season.
Constantly coming up with a big goal or assist, the midfielder was absolutely pivotal in Chelsea's success, and cemented himself among the elite of the EPL.
The Bottom Line
With Shinji Kagawa and Marouane Fellaini not living up to their expectations, and Rooney and van Persie struggling with injuries, United have had no other options than to watch Adnan Januzaj and Antonio Valencia spew in crosses from the wings.
United have had virtually no centralized attacking play this season and signing Mata would address that problem immensely.
While praising his passing ability, it becomes easy to forget that he has an eye for goal as well, with 20 goals in all competitions last season.
Ultimately, Mata is absolutely the high-caliber player that United need, and he would be a fantastic signing.
Make it happen, David Moyes.
Information from ESPNFC was used in this article, unless otherwise stated.