Foster, 27, who faces his former club at St Andrew's tonight, could have been in line to replace Edwin van der Sar as United's No 1.
But Foster is delighted he made his £4million move and said: "I feel I'm back to my best. I feel good and I'm enjoying my football.
"I'm playing week in, week out and I was more than happy to come to St Andrew's.
"A club like United is cut-throat. But I'm loving it (here). I don't wish I'd stuck around at Old Trafford at all."