Kahn was banned from Tuesday's game for improper conduct after throwing his urine sample during a doping control after Bayern's win over Real Madrid in the previous round.
Milan advanced with a 1-0 aggregate victory over Celtic only after Kaka scored three minutes into extra time in the return leg.
While Kaka has scored six goals in the Champions League this season, Milan forwards Alberto Gilardino and Filippo Inzaghi have managed only one goal apiece. Andriy Shevchenko's departure to play for Chelsea this season is still being felt.
Milan is leaning toward a 4-3-2-1 lineup with Kaka and Clarence Seedorf supporting Gilardino as a lone striker.
Gilardino has only one goal in 15 Champions League games in his career, but he scored the equalizer in Milan's 1-1 draw with AS Roma on Saturday in the Italian league.
Three days earlier, Italy coach Roberto Donadoni left Gilardino out for the 2-0 win over Scotland.
"It hasn't been an easy period for me. But you've got to believe in yourself during tough times," Gilardino said. "To improve you've got to be tough mentally."
The other option for coach Carlo Ancelotti is to pair Inzaghi with Gilardino. Inzaghi's 55 goals in 89 European matches put him fifth on UEFA's all-time list.
Ronaldo can't turn out for Milan because he played in the Champions League earlier this season with Real Madrid.
Reserve goalkeeper Michael Rensing will start in place of Kahn and make only his second Champions League appearance.
The 22-year-old Rensing is disgruntled at being a backup and wants to advertise his services to other clubs.
"But I'm blanking that out at the moment," Rensing said. "The importance of this match is very, very high for me."
Bayern midfielder Mark van Bommel is also suspended for the first leg, opening the way for England international Owen Hargreaves to return from a bruised thigh. Hargreaves missed Saturday's 2-0 win against Schalke.
Both Bayern and Milan have struggled in their domestic leagues this season.
While Bayern has improved since Ottmar Hitzfeld took over as coach two months ago, it still is six points behind Schalke to be fourth in the Bundesliga. Milan is sixth in Serie A and trails leader Inter Milan by 35 points.
"This is a huge chance for us. The Champions League is our top objective," Milan defender Dario Simic said.
Alessandro Nesta is back from a five-month shoulder injury layoff, but appeared exhausted after playing the full 90 minutes against Roma. Captain Paolo Maldini sat out the Roma game, but is due to return from a knee injury against Bayern. He's been out of action since the derby loss to Inter on March 11.
"Bayern is always strong physically, but we know them well and I think they can be beat if Milan stays focused," Maldini said.
Milan eliminated Bayern in the second round last season with an emphatic 4-1 victory in the second leg at San Siro, although the German club showed it's capable of winning in Milan by beating Inter 2-0 in September.
Milan is a six-time winner of Europe's top club competition, while Bayern has won four times.