Former Manchester United full-back Gary Neville was amongst those who rubbished De Gea's weak punch that led to Tottenham's injury-time equaliser last weekend.
Ex-Liverpool defender Alan Hansen was also critical of the Spanish youngster, but Ferguson insists the analysis was wide of the mark.
"It has been over the top. It is always over the top when it comes to criticism here," said the Scot.
"You have to listen to some idiots in the game. I am not interested in discussing the criticism because we know it is unfounded.
"Outfield players maybe make 20 mistakes in a game. But they (goalkeepers) are in a crucial position.
"It is unfortunate for the lad but he has to deal with it and we will help him."
That Ferguson felt United should have had a foul in the build-up to the goal only made the situation worse.
It goes some way to explaining his post-match attack on assistant referee Simon Beck, which he was asked to explain by the Football Association.
Ferguson's response was delivered on Thursday and he must now wait until Tuesday before discovering whether any charges are to be brought against him.
The United manager's argument is that he should be allowed to criticise Beck, although he suspects the matter has become personal.
"That is what I have put in my letter exactly. But you never know the FA," he said.
"We are high profile and the profile of me is such that the FA naturally panic as soon as the press criticise them.
"I think that is what you will find. That is why they have sent me a letter.
"Whether I think it is unfair or not doesn't matter to them really at this point.
"I just think it is more about me than what I have said."