Ferguson: Van der Sar will be replaced from within
"Ben has terrific ability and needs game experience. Tomasz is also another good keeper," Ferguson told a news conference.
"At the end of the day the two of them will fight it out, I'm sure of that."
Van der Sar, 39, is due to decide next month whether to carry on playing for another season or bring his career to an end.
The keeper, who has helped United to three Premier League titles and a Champions League triumph during four years at the club, is out of contract at the end of the season.
"Edwin's not said anything yet about what he intends to do," Ferguson said. "He has come back from his recent injury well. It will be Christmas time or sometime in December when he will outline his plans and how long he will go on. We will await that."
CSKA Moscow's goalkeeper Igor Akinfeev had been strongly linked with a move to United.
Foster or Polish international Kuszczak will feature for United in their penultimate Champions League Group B game against Besiktas on Wednesday as Ferguson, whose side have already qualified for the next phase, fields a young side against the Turkish visitors.
Ireland international Darron Gibson will start in midfield while Gabriel Obertan, who signed from Bordeaux in July, will start his first European game for the club against bottom-placed Besiktas.
"We want to win the group," said Ferguson. "It's a good fixture in that we are already through. It means I can try one or two things. We could easily have an absolute non-event game or it could be the absolute opposite."