Mancini was sacked and replaced by De Boer over the summer.
"Inter are a very strange team. There's a lot of talent, but Mancini did not do much fitness work," Gullit told Ziggo Sport.
“The previous Coach knew he'd be leaving anyway, so didn't do very much with the team and left De Boer with players out of shape. Now he needs to find the right way to reconstruct the squad.
“Once De Boer has achieved their fitness, he needs to get them playing as a unit, because Inter are working only as individuals. I hope he can do it, because Serie A is completely different where the strikers are allowed few chances."
Mancini was told about these statements and was interviewed by Radio Incontro Olympia.
“Gullit was fortunate that he was a great player. In football, when you were great on the pitch and then can't do anything off it, it's difficult to accept and at times these people like to talk trash about other Coaches and players.
“It's fine that he is De Boer's friend, but that does not justify certain statements. Gullit isn't the only one, as there are many out there. Jealousy is a terrible thing."