Inter Milan are third, ten points behind second-placed Napoli, with Lazio just three points off the elite European football berths in fifth.
"The need to mediate with someone just to get him to pull on the shirt that they love, that's humiliating," said Spalletti of the ongoing dispute with Icardi.
"Do I need to email 20 lawyers and ask them if I can call someone up?"
Icardi has not played since February 9 after being replaced as club captain by Samir Handanovic amid protracted contract negotiations.
The 26-year-old, who has been linked with a summer move to Real Madrid, had complained of a knee problem but is back training during the week.
However, he remained dropped despite fellow Argentina striker Lautaro Martinez being sidelined through an injury picked up on international duty.
Serbian midfielder Sergej Milinkovic-Savic scored the only goal after 13 minutes heading in a Luis Alberto cross in the San Siro.
"I know what my job is. As things stand today, for the way he behaved, he has to stay out and the others have to play," said Spalletti.
"I have had credibility for 22 years in my career, I have credibility on front of my players.
"People say we lost games without Icardi. Inter didn't get into the Champions League for years with Icardi. Inter lost worse games than this with Icardi.
"(Lionel) Messi and (Cristiano) Ronaldo make the difference, not Icardi, with all due respect.
"Professionalism and self-respect are everything. Discipline is everything."
Lazio had missed out on Champions League football last season at the expense of Inter Milan in the final game of the campaign.