Sanches cancelled out Robert Lewandowski's opener in style in the Euro 2016 quarter-final clash, before stepping up to coolly slot home Portugal's second penalty in a 5-3 shoot-out win on Thursday.
And Fonte insists that United could come to regret their failure to acquire the teenager.
"Being a Premier League player, for me, they missed out on a very good talent," Fonte said.
"He's a nice kid, he listens to the old guys, he wants to learn, which is good. It's great to have him around and he'll be the future of the national side for sure.
"He's got no fear. He just doesn't care. He goes and asks for the ball every second.
"He's got power and strength, energy to run all day and great ability. It is lucky for Bayern Munich that they have him."