Icardi, who has not played since being replaced as club captain by Samir Handanovic and been heavily linked with Real Madrid as a result, is back in training.
There was speculation in the Italian media that Sunday's game at San Siro, which could be key for both sides' Champions League qualification hopes, would see Icardi return.
But while Spalletti hailed the Argentina attacker's improved mentality, the Inter head coach will not be picking Icardi for the match.
"I found him like a new player in some ways," Spalletti told a news conference.
"He trained and we have to put him back into our training behaviour.
"I think it is right that he cannot help his team-mates for this match yet. So tomorrow he is not called.
"He has returned from a long period of absence and there are new things that have happened to be evaluated.
"Then the training with the team is fundamental for me. On the next game, we will evaluate."
Lazio, however, have a game in hand on the teams around them and are only six points behind Milan in the race for Champions League qualification.
"Lazio are a direct competitor for the Champions League and, as in the past, they are having a great season," Spalletti added. "The three points become heavier because they compete with us.
"We must show continuity after the victory in the derby. Our attitude must always be the same, the desire to play the game, to play in a certain way. As we did with Milan we will have to do also against Lazio and Genoa."
Stefan de Vrij and Lautaro Martinez returned carrying injuries after their international duty with Netherlands and Argentina respectively.
"What is fundamental, in my opinion, is that the players who go to the national team have an open dialogue and collaboration with the medical staff of the club and the national teams," Spalletti said.
"We had told the players to highlight and estimate the possibility of measuring the minutes in two games so close. When it comes to young players, who want to demonstrate and contribute to their national teams, it's not easy."