The 38-year-old Dutch international will miss the first two months of the campaign after undergoing surgery on his left hand. He broke his finger saving a penalty from the Croatian Daniel Pranjic during Sir Alex Ferguson's side's 7-6 defeat on penalties to Bayern Munich in the Audi Cup last week.
Foster is likely to be given the chance to deputise for the vastly experienced former Juventus and Ajax player, ahead of Tomasz Kuszczcak, after Ferguson greeted his decision to sign a new, long-term contract at the start of the summer by admitting he sees him as Van der Sar's long-term 'successor'.
Both goalkeepers are likely to play a part in tonight's friendly against Valencia but Ferguson is believed to be ready to give Foster a run in the side, starting with the Community Shield against Chelsea at Wembley on Sunday, to prove his ability to replace Van der Sar when the Dutchman finally retires.
Foster has been widely tipped not just as Old Trafford's first-choice in waiting but as the natural heir to David James for Fabio Capello's England. Rob Green, the West Ham goalkeeper, stood in for the Portsmouth player for June's two World Cup qualifiers against Kazakhstan and Andorra, but Capello is believed to regard his goalkeeping options as limited ahead of the tournament in South Africa next year.
James is now 39 and, while he is likely to travel to the World Cup, cannot be regarded as a long-term option. Doubts linger over Green's command of his box, while Paul Robinson, despite his return earlier in the summer to Capello's squad and the new lease of life granted him by a move to Blackburn last season, has failed to convince the Italian he should be given his place back.
Capello is unconvinced by the merits of the likes of Scott Carson, of West Bromwich Albion, and Joe Hart, now on loan at Birmingham, although he has invited Joe Lewis, the Peterborough and England U21 goalkeeper, to train with the senior squad on several occasions.
The Italian's stated policy of only wishing to select those playing regularly for their clubs has thus far counted against Foster, an eternal understudy at Old Trafford since returning from a loan spell at Watford and recovering from a succession of serious injuries.
The former Stoke goalkeeper, now 26, can no longer be considered a young hopeful and knows he must start to play every week if he is to be included in Capello's 23-man squad