De Boer was dismissed this month after just 85 days in charge, with his replacement Stefano Pioli set for a daunting first match at the helm against Inter's in-form cross-city rivals at San Siro on Sunday.
Argentine Milito, a member of Inter's 2009-10 treble-winning team under Jose Mourinho, believes the Dutchman deserved more time to prove himself.
"I liked him," the Racing Club veteran was quoted as saying by Gazzetta dello Sport.
"I think to sack him has been a mistake. He had winning ideas and could give a new mentality to both the team and club. He just needed more time to work.
"Without good results the coach is [always] the one to pay. But now the players have to take responsibility."
And Sneijder, who joined Inter from Real Madrid in 2009, leaving for Galatasaray under something of a cloud in 2013, also expressed sympathy for the plight of his compatriot, while questioning the direction being taken by the club's new owners.
"De Boer needed more time," he said.
"Frank did good work at Ajax and he did a really good job as assistant coach with the [Dutch] national team. He helped us a lot in the 2010 World Cup. The truth is Inter ended with [former president Massimo] Moratti.
"I am really sad that he left. It's all over now. They need a leader. Someone respected. Where are the Chinese [owners]? I'm sure the players have never met them."
Referring to big-money off-season recruit Gabriel Barbosa, he added: "He [De Boer] did not pick the team. If he would have known that every day they asked him to pick Gabigol because they paid €40 million for him, then maybe he would not have accepted the job."