With United's first-choice keeper, Edwin van der Sar, still sidelined with broken fingers, Ferguson gave Foster a vote of confidence by confirming he would start the Premier League match at Stoke on Saturday.
But Ferguson continues to regard Foster as a better long-term prospect and does not want to risk further undermining the Englishman's confidence after he gifted City their first goal in United's 4-3 win at Old Trafford.
"It was a good opportunity for Tomasz to play on Wednesday but Ben will play tomorrow," Ferguson said Friday. "Ben has not had a lot of game experience but he will be OK."
Stoke provide a tough test for any goalkeeper with their effectiveness at set-plays and the long-throws of Rory Delap requiring good command of the penalty area.
Ferguson expects van der Sar to take another three weeks to complete his recovery from breaking two fingers in a pre-season game against Bayern Munich in July.
Source: AFP European Edition