Make no mistake - Manchester United need a playmaker. Mata could prove a successful one under Moyes but Gundogan may have offered a more concentrated approach - and from deeper on the pitch. The 23-year-old has worked wonders in his time at Borussia Dortmund, while BVB have been nothing close to the same side since the midfielder's back injury.
Gundogan also has the advantage of being two years younger than Mata, while his addition would have finally provided a remedy to United's long-standing central midfield problems. The German's current fitness issues mean he wouldn't have had an immediate impact at Old Trafford, however.
Recent reports have suggested that David Moyes does not consider Bayern Munich defender Dante to be good enough for a move to Old Trafford. Worries have also supposedly been expressed about the Brazilian's age (30).
But with United's defensive woes this season, can Moyes really afford to splash the cash on a creative midfielder when his back four is full of instability?
Veterans Nemanja Vidic and Rio Ferdinand, for example, look to be heading out of Old Trafford this summer, while their collective fitness and performance levels do not inspire hope before the end of the current campaign. Questions, too, can be asked of Chris Smalling, who has struggled during the important stages of United's season. A new defender could have - and may still - prove crucial.
With nine assists and three goals in La Liga this season, Koke has been instrumental in Atletico Madrid's charge towards the summit of the Spanish league. And at 22 years of age, the Spaniard - tipped to make Spain's World Cup squad - clearly has a bright future ahead of him.
Koke has this month signed a new contract at Atletico, dealing a huge blow to United's hopes of luring him to England, but the Spaniard could surely have been persuaded to move to Old Trafford before he signed a new deal had enough been put on the table.
Remember the last time a young talent burst onto the scene at Everton and drew interest from Manchester United... That Wayne Rooney didn't turn out so bad, did he?
And it could have been a similar scenario for United this January if they had snapped up Everton midfielder Barkley as soon as the opportunity presented itself. At 20-years-old, Barkley is brimming with potential, while Moyes' connection with him from his time at Goodison Park could have inspired him even further.
Barkley could also have contributed to United's Champions League campaign as one of the few transfer targets out there who is not cuptied in Europe. As things stand, though, any future bid will likely rise in price the longer United leave it.
So, while Mata's quality is no doubt capable of inspiring Man United to new heights, other targets may have plugged more pressing gaps within the squad at Old Trafford.
The only question now is whether the Glazers give Moyes the money to pursue these buys on top of the big-money purchase of Mata.
image: © Jason Gulledge