The Anfield hero is fuming with Benitez for splashing out £248million and leaving the club in a mess - a "sinking ship".
Grobbelaar said: "The Liverpool ship has been rocking because the person who left has spent £248m and look at the players we have got.
"As a Liverpool fan, I'm really sick that this has happened. He's wasted money and then turns around and says you have to pay another £20m to get rid of him? I think that's ludicrous.
"Why? Because he wanted his backroom staff to go with him. Well, he's now got his backroom staff at Inter Milan. So cheerio. Bye-bye. And good luck. I'm very angry at what he's done to my club."
Grobbelaar, who made 440 appearances in 14 years at Anfield, is also unhappy about how Benitez conducted himself last season.
He said: "I'm angry when he stands up and says he can get fourth place. They should have fired him when that became mathematically impossible.
"He was that confident and that brash about what he could do. And when he doesn't do it it's a smack in the face.
"There's a huge hole in the ship and we have to block that hole."