Speaking at the New York launch of Tour 2010, the boss said: "I think Ben deserves the opportunity to do better for himself. That's the reason we accepted the offer. I still feel very strongly about this... I think he's the best English goalkeeper in England."
News sources in England were reporting on Tuesday that the 27 year-old was expected to complete a medical and agree personal terms before the end of the week. Foster joined United in July 2005 but has made just 23 appearances for the club.
United have been linked with a number of established goalkeepers from the European leagues in recent weeks, but Sir Alex told reporters in New York that he would place his faith in young stopper Ben Amos.
Amos, 20, has made just one first-team appearance (vs Middlesbrough in the Carling Cup third round in 2008/09), although he has consistently impressed for Ole Gunnar Solskjaer's reserve team this term.
Sir Alex said: "When we decided to let Ben [Foster] go, we were in the process of letting a good young goalkeeper, Ben Amos, go out on loan. He's in the England Under-21 team but we can only accommodate three keepers. So when the situation with Ben [Foster]'s future at United came up and we agreed to let him go, we stopped Ben Amos going on loan.
"He will now be our third keeper. He's very talented and has a good future ahead of him."