Foster insists he has no axe to grind with United but that a parting of the ways was inevitable after he started only one Premier League game - against West Ham - after November.
He told the Sunday Mercury: "I'd been at United five years. I really enjoyed my time there. I'd been out on loan for a few different teams, spent a couple of years at Watford, and seen that side of football.
"To be at Manchester United is a different world and the last couple of years have been at up and down for me, in and out of the team.
"You come to a point in your career when you realise it's not going for you at a certain club and that time had come for me.
"I was delighted to join Birmingham. It's a team with ambition, a team looking to continue on from last season's progress when they did excellently to finish ninth."