Beat German champions Borussia Dortmund and City can dare to dream. But anything less will induce nightmares on the prospect of another early exit from Europe's elite.
"Yes, we need time to improve in the Champions League, but we do not have time," said Mancini.
"We have only five matches and we cannot concede goals like we did against Real Madrid. We have to improve quickly.
"It is important to get into the second stage, because after that anything can happen because we are a good team."