Boothroyd believes Foster will become the world’s best keeper. Foster has been at Vicarage Road on a two-year loan spell from Manchester United.
He said: “Ben is a terrific player. He was injured for the 3-0 defeat by Everton and when you lose a player of his quality you really miss him.
“He has such a presence and an ability to start attacks, whether it’s on a counter-attack with his long throws or whether it will be lofting a ball down the side for somebody to run onto.
“Even when the back pass comes to him from one side, he can drop the ball onto somebody’s feet on the other side. He has exceptional talent and we are going to miss that.
“If you asked him he’d tell you he could play centre forward or left wing but I think he’s best where he is. He’s part of the team and has a great humility about him.
“He recognises the fact he’s just a cog in the wheel. He works harder at his game and that’s why I think he’ll be a future Manchester United and England number one.
“I’m sure he will be back at Manchester United next season. He’s better than Edwin van der Sar in my opinion, but then I’m not picking the Man Utd team.
“He is going to be the best goalkeeper in the world, I’m convinced of that.”