The Red Devils shot-stopper is under intense scrutiny after several high profile errors this season.
De Gea arguably cost his side two goals in a 3-1 FA Cup semi final loss to Chelsea on Sunday.
And Ferdinand thinks it would take a big money move to find someone to fill De Gea's shoes.
He told BT Sport: "He [De Gea] has had a bad spell, confidence is low, but does that mean you go out and get rid of him? Who are you gonna get?
"Who is out there who is actually able to be bought right now and who is going to be a world-class goalkeeper right now at Manchester United?
"The only one I see is Oblak, but can you get him out of Atletico Madrid? And if you do how much are you going to have to spend?
"An obscene amount of money. I say stick with him [De Gea], he has earned that right to be stuck with."