Mustafi joined Everton from the youth set-up of German club Hamburg in 2009, going straight into the academy at Finch Farm.
He was a regular for the reserve side during that time as well as making a senior appearance in the Europa League against BATE Borisov in December 2009.
However, the big German defender struggled to break into the first-team set up under David Moyes and failed to make another appearance before being released in January 2012.
Since then though his career has been on a sharp upward trajectory. He signed for Italian side Sampdoria, helping the club earn promotion to Serie A at the end of the 2011/12 season.
He was a regular in the side upon their return to the top flight, eventually earning a move to Spanish side Valencia in the summer of 2014.
By that stage he was a full Germany international having played at every youth level from Under-17 through to the senior side, picking up a World Cup winners' medal in the process.
Now it appears he is on the verge of taking an even greater step, with Spain's top two sides reportedly vying for his signature - according to his agent Ali Bulut, who told German daily Bild:
"There have been direct enquiries from Real Madrid. But no further conversations nor negotiations.
"Barcelona have also been in touch with us with a view to incorporating Mustafi in the winter transfer window. [But] Mustafi has a good contract at Valencia, a great club and for him to leave Mestalla, everything would need to be perfect."
In Everton's defence, predicting the potential of a player is not an exact science - this isn't Football Manager. Sometimes a player needs a move in order thrive and that may well be the case with Mustafi.
Should the 23-year-old pull on a Real Madrid or Barca shirt next season there will almost certainly be a touch of pride from those inside the Goodison Academy who helped his development as a youngster. However, there should also be a touch of regret at allowing an international-class defender slip through their fingers, especially when the club is looking to replace Sylvain Distin, Antolin Alcaraz and eventually club captain Phil Jagielka.