The Portuguese defensive midfielder, 21, will join the 2012-13 Premier League champions after the 2014 FIFA World Cup, where he is expected to play a part in Paulo Bento's plans.
So who is William and why is he costing David Moyes so much? We scout the young talent and provide a verdict.
Sporting are remarkably proud of their all-Portuguese, all-homegrown midfield on show in every Liga Sagres game.
Andre Martins and Adrien Silva are well-established professionals in their mid-20s, but William is already the finest talent they house and glues the midfield together despite being much younger.
The Leoes are still recovering from several years of financial mismanagement and luxury buys who flopped, so a focus on developing youth has become key to their sustaining of a challenge at the top of the league.
Over the last 18 months, William has made great strides with every game played. He's developed physically, mentally and technically at the same rate and looks a complete midfield prospect.
The Anatomy of William
William is one of the finest young defensive midfielders emerging in the game, with his incredible combination of size, somatotype, range and instincts comparable to the best.
He measures in at just over six feet and boasts extreme core strength. This allows him to physically dominate most opposing No. 10s who enter his zone—both aerially and on the deck—and plays reliably as a sole holder in a basic 4-3-3 for Sporting.
Centre-backs are happy to hand him the ball when playing out in tight spots, as his massive frame shields him from tackles superbly and allows him to become an easy conduit for the ball to move through.
Off the ball, he boasts great ability to drop into space, right a lopsided formation and cover holes his attacking runners leave. His immense range is Morgan Schneiderlin-esque, and he's able to play a sideline-to-sideline covering role and keep the team tight at the back.
Cedric Soares, at right-back, has had a breakout year thanks to the solidity William gives Sporting's system, and he is now pressing Joao Pereira to start for the national team.
The defensive midfield monster has an outside chance of starting for Portugal at the World Cup, and Os Seleccao fans are relieved their country has finally produced a true holder to anchor the formation. Miguel Veloso was never a natural fit, and William has a shot this summer.
Over the coming weeks, you're going to see a lot of comparisons to current players when speaking of William. In a way, he has a bit of everyone, and likenings to Yaya Toure, Sergio Busquets and Nemanja Matic are all valid in their own way.
That said, he is most like Schneiderlin or Busquets until his attacking game develops. He has raw defensive instincts, he's a natural at the position, but he's not the box-to-box Toure presence every fan yearns for just yet.
If he can refine his defensive game, the comparisons to Matic will be undeniable. He'll be a stronger version of the Serbian in no time.
But for now, he's a careful, considered player who does the basics right and doesn't try to complicate things. You may ask why a player such as this costs £37 million, and the reason is because he's physically blessed and potentially capable of much, much more.
Like Eden Hazard and Jose Mourinho, William needs an excellent coach to take his game to the next level. He's nowhere near the finished product, but he shows innate instincts the likes of Mikel Arteta could only wish for.
It's a gamble, and it's a lot, but there's no questioning his potential. Many will turn their noses up at the fee, but is he worth £7 million more than Fernandinho? He may well be in three years.