"Ben's at one of the biggest football clubs in the world," said the former England keeper, who was speaking at the launch of the Nationwide Football Annual.
"To be an international goalkeeper it helps if you're playing at the very top level. Now he's obviously looked at the situation with Van der Sar, a world-class keeper coming towards the end of his career but still performing well, and feels that taking that four-year contract he's going to have the chance this season to play in the first team.
"Whether it's on a regular basis or two or three times a month, I don't know. But he's obviously had conversations with Manchester United where he feels that is the place to be."
Clemence says he has spoken to Foster about the international situation, and while he would not divulge details of those conversations, he did accept Foster, or any goalkeeper, should be playing regular first-team football.
"It's a concern because you want your keeper to get as much experience as he possibly can, and unfortunately part of a keeper's learning experience, particularly when young, is learning from your mistakes and learning from mistakes at the highest level," Clemence said.
"It's how you cope with those, and how you bounce back from them. So therefore Ben playing at that level on a more regular basis is only going to help him.
"In an ideal world you want all of your keepers playing every single week but you don't live in an ideal world. [But] I saw Ben for two years at Watford [on loan, 2005-07] and I know what he's capable of. And obviously he's been in the squad and I've trained with him. Not just me but everyone who's worked with him is impressed."