After six weeks of staying away and negotiations with the club via a lawyer, Icardi returned to the team last week.
“Everyone in a team is important, but nobody is indispensable," Nainggolan told the Corriere dello Sport.
“Mauro is a great player, but we have another like Lautaro Martinez who did very well when given the chance and is destined for a future at the top of the sport.
“Too much has been said. If Icardi had problems with someone, it should've been up to him to resolve them and not let those issues damage the rest of the team.
“Enough now, he is training and available, just like everyone else. If he continues like this and scores goals, then he is an important player to have."
He added: “I told him what I thought, which was that if he had problems that he needed to resolve with someone, the whole group could not be made to pay for it with his absence.
“Spalletti put the focus on the group. A Coach must be credible in the eyes of his players and Spalletti tried to deal with the situation as best he could by doing what he had to do.
“Even if Spalletti wanted me here at Inter, I paid the price when I made a mistake. That's how it has to work. For everyone."