Milan coach Carlo Ancelotti has voiced his concerns after a number of key players picked up injuries ahead of the clash with Arsenal.
Alessandro Nesta, Andrea Pirlo and Kaka all missed Saturday's 1-1 draw against Lazio, while Clarence Seedorf was forced off with a muscle problem and Ronaldo is a long-term absentee.
Milan held Arsenal to a goalless draw at Emirates Stadium in the first leg of their UEFA Champions League tie, but Ancelotti admits he is anxious about Tuesday's return match.
"The match with Arsenal is definitely causing me worries because we're not coming into it in optimal conditions," he said.
"Seedorf has a muscle problem. We'll see but it will be very difficult for him to recover."
Ancelotti is nevertheless confident that those players still available to him will dig deep and find something extra.
He explained: "It's a match in which players find energy because it's a question of character, courage and desire to get to the next round.
"That's what will make the difference rather than physical condition.
"We're tired but at this time of the year all the teams are.
"We felt the same way last year but we got through to the next round and went all the way (winning the Champions League)."