"Obviously we have signed David de Gea as the main choice, I would imagine," said Ferguson.
"But Anders Lindegaard and Ben Amos have probably said to themselves they have an opportunity with Van der Sar retiring."
Former Atletico Madrid keeper de Gea has been assured of minutes in Saturday's friendly against Chicago.
"De Gea will play on Saturday against Chicago but they will all get their fair share of games and the decision on who starts the season will depend on how de Gea settles," added Ferguson.
The 20-year-old Spain under-21 international signed for United in a deal reported to be around £19m earlier this summer.