Lukaku was sidelined with a thigh injury in August and is close to returning but the trip to Florence comes too soon for the Belgium international.
"Lukaku has been training partially with the group for two days, he clearly needs to regain match fitness but is very motivated," Inzaghi told Inter TV.
"Finally, after a long time, that lesion has healed, he won't be with us tomorrow, but we hope to have him by Wednesday (Viktoria Plzen in the Champions League). Seeing him work, I am feeling confident."
The Italian coach said that goalkeeper Samir Handanovic (38), midfielders Marcelo Brozovic (29) and Roberto Gagliardini (28) are also sidelined and will not be available for Saturday's match.
Inter have had a difficult start to the season, as they sit seventh in the league standings.
However, three wins from their last four games, including one over Barcelona in the Champions League, have put them back on track. Inzaghi still expects a tough clash against Vincenzo Italiano's Fiorentina, who are 13th in the standings.
"Florence is historically a difficult place for anyone to play in, but fortunately, we managed to work well in training. Results bring self-confidence and we know that we'll be under pressure every three days from now until the break for the World Cup," Inzaghi said.
Inter have five league fixtures as well as two Champions League games, including Bayern Munich away, before the World Cup begins in Qatar on November 20th.
"We have to take it one game at a time and there are many before the break for the World Cup. We've got to look at the fixture list and be realistic.
"We've dropped a few points, but the objective is to close the gap and get through in the Champions League," Inzaghi added.
