Icardi was stripped of the captaincy in February after being embroiled in protracted negotiations over the renewal of his contract and the 26-year-old then said he could not play because of a knee problem.
Icardi, Inter’s leading scorer in each of the last four seasons, returned to training this week after a meeting between his lawyer, Paolo Nicoletti and the club.
He was expected to face Lazio but was surprisingly left out by Spalletti.
“To have to negotiate with a player to get him to play and wear the Inter shirt is humiliating, for the fans and his team mates,” Spalletti said after third-placed Inter’s 1-0 defeat.
“Do I have to email his lawyers before I can select him?”
“Today, Mauro Icardi had to be left out because of the way he has behaved, it’s only fair that the others play… It’s a question of credibility.”
Spalletti said “the events have happened in front of everyone and are easy to interpret.
“You must have respect and the right behaviour in the dressing room. The coach must be credible and recognised as someone who will do the right thing.”
Spalletti added that Sunday’s defeat by Lazio after Sergej Milinkovic-Savic’s early goal was not due to Icardi’s absence.
“How many defeats, worse than this one, have we had with Icardi on the pitch?” he said.
“Lionel Messi and Cristiano Ronaldo make a difference but there are very few like them. Professionalism and self-respect are everything. Discipline is everything.”