While news of United scouting the Sporting Lisbon prospect caused barely a ripple in the British press, David Moyes' informal enquiries only confirms what those inside Portuguese football have been saying since William was the subject of a transfer tug-of-war as a 13 year-old.
Now 21, the Angolan-born William has been involved in Portugal's national teams since he was 15. But it's only this season that he's carving out a place in Sporting's first team.
Last term was spent in Belgium with Cercle Brugge, while the previous season he played in the Portuguese Second Division for Fatima.
But after winning back-to-back Young Player of the Month awards this year, William's emergence is a great triumph for Sporting's youth development.
And the recognition that has come with his breakthrough, where United have been joined by Barcelona and Arsenal in posting scouts to watch the youngster, is no surprise to Fatima manager Ricardo Moura.
"He will be the next No6 for the national team. William is demonstrating his full value and will soon assert himself at international level," said Moura.
"It's not easy to find such a complete player for that position."
Moura goes further, likening William to Arsenal legend Patrick Vieira, only "technically more evolved".
"He can also play in an advanced midfield position. He's good with short and long passing on the left and right foot. He's good in the air. He is very complete."
Sporting president Bruno de Carvalho knows they have something special on their hands and he made it a priority to secure William to a new contract before the start of the season. Now tied to Sporting until 2018, the midfielder's buyout clause is set at €45 million.
"There's no doubts about William," said De Carvalho.
"With time, no one doubts that William will earn his place in the Portugal squad.
"We'll help him grow step by step. When he is called into the national team, he'll be there to stay."
Even if De Carvalho does choose to cash in, it will still take something special to convince William to leave - as Benfica discovered almost eight years ago.
Then, as a 13 year-old, William was in his first season with Sporting and not yet formally signed.
Having been alerted to this prodigious midfield talent, Benfica rolled out the red carpet for William and his family. A club delegation visited the Carvalho home, armed with a contract for William to sign.
But with no less than five senior representatives staring back at him, William said "No".
A former junior coach of William's recalled: "We realised that something was wrong: we called him to the side and asked him why he would not sign. This was what every kid wanted, but he was not convinced.
"He was always a Sportinguista and had the dream of playing for Sporting. We had the difficult task of explaining this to Benfica."
But there was still some work to do for Sporting to be sure of William's commitment. And this is where United could have an advantage on any bidding rivals.
Knowing he was a Sportinguista, club officials asked William who his favourite player was.
"Nani," came the reply.
Within days, Sporting's board had organised for the United winger to call William and that was more than enough to secure his commitment.
Eight years on and William is now on the brink of playing with his boyhood idol for his country - and possibly, should things go to plan for Moyes, at club level too.
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