Foster has endured mixed fortunes since stepping in for Van der Sar when the veteran Dutchman broke two fingers during a pre-season encounter with Bayern Munich. There have been times when the 26-year-old has excelled, notably at Wigan and Stoke, where saves at crucial times provided United with the platform for victory.
However, Foster has made some high profile errors which have cast doubt over his ability to become a long-term successor tp Van der Sar. Foster made two mistakes against Manchester City and another a fortnight ago when Kenwyne Jones shrugged him aside to score Sunderland's second in the 2-2 draw at Old Trafford. On both occasions, the results could have been far more serious had United not netted late goals.
Foster's most recent experience is a better one, keeping a clean sheet for England on Wednesday as he collected his third cap in the win over Belarus. In confirming it will be Foster's last high-profile game for a while, Ferguson insists his faith in the former Watford man is undiminished.
"I still feel he is the best English goalkeeper around,'' the United boss said. "He has been outstanding in a lot of things. The save he made on Wednesday was typical of his performance level with us too. They are a demonstration of the guy's ability.
"But he still has not had a lot of game time and experience. His injuries have curtailed the progress he was making. He is only going to get better, though, and there is no question he will be a fantastic goalkeeper.''
He is evidently not fantastic enough yet, though, for Ferguson to take even the merest second to consider whether fit-again Van der Sar should make an instant comeback.
After a reserve team outing at Everton last week confirmed his return to fitness, Van der Sar always felt confident of being recalled for the start of a week that also includes a Champions League trip to CSKA Moscow a trip to Anfield to take on fierce rivals Liverpool.
"Edwin is fit and fine and he will play,'' said Ferguson. "He has got the experience. It is fair to say he and Peter Schmeichel have been the best goalkeepers in this club's history.
"With that calming influence he has on anyone around about him, getting him back is important - and with games against CSKA Moscow and Liverpool coming up, he couldn't be coming back at a better time.''
With Van der Sar hinting that this may not, as had been expected, be his last top- flight season, Ferguson may have a little more time to decide whether Foster can be a reliable first-choice goalkeeper for the Red Devils.
At the moment, he must wait to see whether he is called upon for the Carling Cup trip to Barnsley on October 27, with fellow understudy Tomasz Kuszczak also desperate to play.
Van der Sar will need to be on his toes to deal with Bolton's aerial presence but despite the absence of Wayne Rooney and possibly Darren Fletcher and Dimitar Berbatov, Sir Alex Ferguson's side will be hoping they won't be relying on a late goal to rescue them this time out.