Ferguson said United were already planning for life without van der Sar on the pitch, although the 40-year-old Holland international could possibly stay on as a coach at Old Trafford.
"We are planning for this being his last season," Ferguson said. Asked about whether the goalkeeper could stay on to help coach his successor, Ferguson said the issue had not been discussed.
"But Edwin is a player who would be of interest in terms of his knowledge and standing in the game," he told reporters.
United last week confirmed the capture of Anders Lindegaard from Aalesunds FK on a three-and-a-half year deal for an undisclosed fee.
The 26-year-old is already training with United and will be formally registered when the transfer window opens in January.
"We can't register (Lindegaard) until after the Stoke game because of the Bank Holiday," Ferguson added.
"But he is training with us now and that is important. It will bring him to a better level of fitness because he has not played for a few weeks. We will see how his progress goes."
United will go into Boxing Day’s encounter with Sunderland with a squad that has been riddled with flu in recent weeks.
The virus would have ruled Nemanja Vidic and Anderson out of last weekend's trip to Chelsea, if it had not been postponed while Michael Carrick and Nani have both also been struggling.
That is the last thing Ferguson needs ahead of a contest with the ever improving north-east side, managed by United’s former captain Steve Bruce.
"The biggest thing we have been trying to do is clear up some flu that is around the place," Ferguson said.
"We have had bits of it over the last eight days and Vidic and Anderson have both had it so that is interesting, Carrick and Nani have both also been down with it but hopefully they will be ready for Sunday. They are all training now.
"We were lucky to get a point at Sunderland. It is the hardest game we have had away from home all season.
"I think it is a tough league and I think Sunderland are a perfect example.
"A few weeks ago they were at the bottom of the league and now they are challenging for a Europa League place. If you win a few games you can put yourselves back in there because it’s so tight."
Ferguson expressed "surprise" that others are questioning just how tough the Premier League is this year and just how congested the top of the table is.
However, he maintains that he will be delighted if United can still claim the top spot once the festive period is finished.
"My aim is to try and be top by 4 January. That would be good," he said.
"If we are, we have done well. By that time, the top of the league will have taken shape, depending on the weather."