The English club are staring at the prospect of a quarter-final exit, after winning the competition last season.
They trail Real 3-1 after the first leg at home, and must win by two goals to force extra time, or win by three goals to close out the game in 90 minutes.
"It is one of the biggest challenges to perform away at the Bernabeu," said Tuchel in his press conference.
"It is even more difficult if you have to earn a certain result, if you need to win with a minimum of two goals or better or maybe a three-goal difference. That makes it almost impossible and very, very difficult, but still it is worth trying.
"We need nothing else than a fantastic script that we want to be able to overcome this."
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