Tomasz Kuszczak had been tipped to step in for his first Premier League start of the season after he kept a clean sheet in the Carling Cup win over Wolves on Wednesday.
But Ferguson will stick with Foster, who is currently deputising for veteran Dutchman Edwin van der Sar.
Van der Sar suffered two broken fingers in a pre-season game against Bayern Munich in July and will be out of action for another three weeks.
Foster began the season as United's number one but has come under fire for his performances against Chelsea in the Community Shield and Arsenal at Old Trafford, while his mistakes against Manchester City had expected to result in Kuszczak starting at the Britannia Stadium on Saturday.
Meanwhile, United midfielder Owen Hargreaves could be back playing in four weeks.
Hargreaves has been out of action for a year after deciding to undergo operations on both knees in an effort to solve his tendinitis problems.
The 28-year-old has been doing his rehabilitation work in the United States so far under the guidance of renowned knee surgeon Dr Richard Steadman.
He is now back with us and hopefully the boy gets a break and continues his progress without any further mishaps
Sir Alex Ferguson
There have been doubts about Hargreaves' long-term future but now he is back in Manchester and could be set for a return to football.
"I feel it will still take us four weeks to get him into the rhythm of training again and get to the point of match play," said Ferguson.
"But he has the determination and resilience to see it through. He is now back with us and hopefully the boy gets a break and continues his progress without any further mishaps."
And Ferguson has also taken comfort from the growing number of players that have come back from formerly career-threatening cruciate knee ligament surgery, the latest being Chelsea's Joe Cole.
"We don't see the fact he has been out a year as a handicap," said Ferguson.
"Other players have been out a similar length of time.
"When Joe Cole came back on Wednesday you forget he was missing for nine months. It is achievable, particularly when they are young.
"Owen is not kicking a ball yet. The physio who attended to him over there will stay here for a week for the changeover.
"He and our physios are discussing exactly at what stage he is at but he should be into football training quite soon."
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