"After the [3-0] defeat by Malaga we are in a very uncomfortable position," said Spalletti. "We need to win to restore the confidence that will bring out our stronger qualities.
"Milan are enduring the same loss of form that we are, and for both teams this meeting is critical for morale and even fundamental for the future of both.
"Zenit are better than a side with two victories in eight matches. My players have real character, though."
In the other Group C game on Wednesday, Anderlecht will host Malaga.