The Black Cats sit at the foot of the Premier League table with just two wins from seven matches, with the pressure mounting on the Dutchman.
But striker Fletcher says Advocaat has the backing of the playing group as they strive to get themselves out of their early-season mire.
"We do back him, not that he needs it. He is a very experienced manager. He knows what he is doing and I am sure we will get the result we need," said the Scotland international.
"The gaffer is working hard for us. He is the same, the way he always is. He is not going to change no matter what has gone on. He is a strong man as you can tell by looking at him.
"There is a great bunch of lads here so we will keep upbeat and hopefully the win will come sooner or later."