Foster, who looks poised to complete a £6million switch from United to St Andrew's, told the Daily Express: "You never want to leave Man Utd but I want to be playing. I'm not interested in just hanging about."
Hart admits he wanted to stay at Blues but has been recalled by his parent club Manchester City after a successful season-long loan with Alex McLeish's side.
But he has no doubts that Foster, starved of first-team opportunities at Old Trafford, will be an ideal replacement.
Hart said: "I think Ben will do it easily (follow me). He is a fantastic keeper.
"Ben has had some harsh criticism at Manchester United this season but, if he does turn out to be the Birmingham keeper next season, he will do a good job."
Hart wanted to remain at Birmingham but accepts the final say was with Manchester City.
He said: "I just waited for my orders, and my orders were that I went back.
"I am contracted to Manchester City so that's where I will be.
"I will admit, it's sad. But it was out of my hands. If I could have stayed at Birmingham, I would, but it wasn't down to me.
"I didn't really want to leave but I was told that I have to. It's just the way football is."