McLeish revealed Sir Alex Ferguson told him Foster would be back in the England team by the end of the campaign when completing the £6million switch from Old Trafford.
Foster has been a model of consistency and produced what he described as the best save of his career to deny Carlton Cole during Saturday's 2-2 draw with West Ham at St Andrew's.
McLeish said: "That was a fantastic save, close in, point blank, and his reaction was wonderful. He had agility, the speed with which he got across was fantastic.
"You have to be in tip-top form to be able to move like that and Ben is in top form.
"You can see from that save and the penalty save he made earlier in the season that his form is spectacular.
"I knew we were getting a guy who just needed to play and even Sir Alex said to me when we signed him 'he will be in the England team by the end of the season'.
"He is fighting Joe Hart for that position. He is a good 'keeper."
McLeish added: "If a 'keeper makes a mistake it is normally fatal but Ben has learned a lot from his Manchester United days and his loan spell at Watford. He is looking a very accomplished goalkeeper.
"It was great we were able to do a swift deal with Manchester United.
"At the time, everyone else was okay for 'keepers so we had no real competitors.
"A new 'keeper was top of the list after Joe Hart did so well last season and Sir Alex couldn't hold onto him any longer.
"He had to be fair to the guy and he has done him a favour there."
Birmingham have won only twice this season in the Barclays Premier League and McLeish knows there has to be improvements in defence and up front.
He said: "I am not too unhappy with the performances. Results-wise I believe we should be three or four points better off but I guess any manager in the Premier League will probably say that.
"I feel we have uncharacteristically lost some goals which we shouldn't lose because the one thing we prided ourselves on last season was the defensive aspect of things.
"The guys got a lot of shut-outs and it is time to get back on that sort of trail as well.
"But we've also got to be more of a threat at the other end of the pitch to take a bit of the pressure off the defenders."
McLeish is considering handing a start to Chile winger Jean Beausejour after recovering from his knee problem.
The former Scotland boss said: "He lacks match-fitness but when is he going to get match-fitness? I have to consider him.
"He is the one out-and-out winger we have and we have to change the dynamics of the team to get the service to Cameron Jerome and Nikola Zigic.
"If they are in the team, that's his type of game.
"We will try and get him as much involved in the action as possible over the coming weeks."
McLeish has no fresh injury concerns ahead of tomorrow's match with Tony Pulis' side.
Colin Doyle will continue to act as back-up for Foster while Maik Taylor nurses a shoulder injury which caused him to be ruled out of the weekend draw with West Ham.