Ferguson: I had to let Foster go
The 27-year-old goalkeeper completed a move from Old Trafford to Birmingham City on Tuesday, despite Ferguson believing he is England's No.1 shot-stopper.
"We accepted the bid from Birmingham City simply because we knew how concerned Ben was about his future and I can understand that," Ferguson said in New York on Monday.
Foster started the first eight league games of the season for United after first choice gloveman Edwin van der Sar recovered from an injury picked up in pre-season.
And although Foster won two England caps as a result, the former Watford loanee then featured in just a single game since November, costing him a place in Fabio Capello's squad for the World Cup finals.
"I think most of that comes from his disappointment of not having played enough games to get in the England squad this summer," Ferguson added.
"I still feel very strongly about this, that Ben's the best England goalkeeper and I can't work out why he wasn't even considered simply because he didn't have enough football.
"Sometimes your conscience does come into it and you have to understand a player's feelings about it.
“Ben's been a terrific servant for us, he's been very unlucky with a couple of injuries he's had with us last season and I think he deserves the opportunity to do better for himself."
England under-21 custodian Ben Amos will now be promoted to the United first team squad in place of Foster.
By Owen Edwards