Former Ajax boss de Boer was given his marching orders after just four games of the Premier League campaign following a 1-0 loss at the hands of Burnley.
Roy Hodgson was given the task of replacing the Dutchman but has still been unable to pick up a single point in the league in his three games at the helm.
Ronald de Boer has not been impressed with the Eagles' decision and feels his brother should have been given more time to get his philosophy across to the players.
He told BeInSPORT: “It hurts, definitely. He’s a winner and he worked so hard at Crystal Palace to get it going. Finally, when he thought there was a light at the end of the tunnel he got sacked so that hurts even more.
“He has to stand up again. He’s still hungry to coach another team but for sure it was a knock down.
“We just started, that’s why it’s so ridiculous [to be sacked] after four games. They played a good game against Burnley.
“You thought they’d begun to understand a little bit the philosophy, the way. It takes time."