The 46-year-old believes Jose Mourinho's men could pull off a comeback if they start the match with attacking impetus and are backed by the Bernabeu
The Spanish powerhouse were beaten 4-1 in the first leg at the Signal Iduna Park last week, but the former Germany international believes they can turn things around in the return if they can score an early goal.
"Tactics won't play a huge part in the outcome of the tie but the first 15 minutes will be extremely important. Madrid must attack from the first minute and play with intent," he was quoted as saying by Marca.
"If they manage to do that, the fans will respond and it will be a great atmosphere, something which has played a huge role in some special Bernabeu moments in the past.
"It will be difficult but nothing is impossible."
Illgner represented Real Madrid between 1996 and 2001.