Foster has made a £6million switch to Birmingham after missing out on Fabio Capello's squad for the World Cup finals this summer.
He had effectively become third choice at United - behind Van Der Sar and Tomasz Kuszczak.
But the 27-year-old is looking to make the same impact for Birmingham as Joe Hart, who is heading to South Africa with England after a successful season on loan from Manchester City.
Foster said: "I know I was at Manchester United, the biggest club in the world, but I don't enjoy not playing regular first-team football.
"Training is all well and good but matches do it for me. In the last two or three years I've played 20-odd games and it is no good and to be fair to Edwin (van der Sar) he can go on for another two or three years.
"He is that fit. I think the main concern was, when Edwin signed his new deal, everyone knew he is going to be the number one 'keeper next season.
"He could go on for another few years after that as well. He is a fantastic keeper. I'm 27, I just need first-team football now, enough is enough and it is time to move on."
Foster admitted: "The lack of football has cost me at the end of the day in terms of England.
"I am not on the plane to South Africa and I can't argue with the manager's decision. I've played one (first-team) game in seven months.
"I've seen Mr Capello at few matches with United this season and he has asked me what is going on and things like that.
"I think he will be fairly pleased that I am looking to play some first-team football now.
"I'll get behind the country now like the rest of the fans and hope we do really well in the tournament.
"For me the aim is to well with Birmingham and get back with England for the next European Championships.
"The manager (Capello) did play me in a few games so he did see something in me that he quite likes so, with the chance to play first-team football, hopefully I'll be able to get back in there again with England."
Foster concedes he has a hard act to follow in Hart but is ready for the challenge.
He said: "Joe did fantastically well last season. He is a fantastic 'keeper. To be honest, I never expected anything other than that from Joe.
"I've got to follow on from that. They are fairly big gloves to fill but I am confident in my own ability and pretty confident I can do it as well.
"I didn't speak to him directly before coming to Birmingham but, when I was with the England squad and Joe was involved, he just told me about the club, the goalkeeping coach, the manager, things like that, what the lads are like.
"He only had positive things to say, how much of a good time he used to have, and you saw he didn't really want to go back to Manchester City.
"He wanted to stay for another season at Birmingham which is understandable because he has had a good year this year and you just want to be playing first-team football."