The Swede was again on the scoresheet in their Champions League victory over Auxerre.
"It would be tricky without Ibrahimovic, as we don't have someone of the same characteristics," confessed Allegri. "We could play Ronaldinho and Robinho together for a side with great technique, but less physical presence.
"This qualification gives us strength and it is an important night for us. I am also very glad that we kept a clean sheet for the third time.
"We are solid, don't allow our opponents much and create at least four or five clear-cut chances per game."