The Spanish side face a mammoth task to make it to the final at Wembley, after the Pole hit all four in a memorable evening at the Westfalenstadion. 
Mourinho believes his players could have done much more to limit the striker, but has backed his side to turn the 4-1 first-leg result around in Tuesday's return game.
"In the first leg, we didn't even foul Lewandowski once. We were way too naive and pure," the Portuguese said.

"We have to play it minute by minute and goal by goal until the last breath of the match.

"I respect Real Madrid's history a lot. It is difficult to say what mathematical chance we have. I feel this is the most important match in 10 years for Real Madrid, but I also felt that in Dortmund and we played like [it was] a friendly.

"It's difficult to say what mathematical chance we've got. I've already said everything to my players. The most important thing was to discuss the mistakes we made in Dortmund.
"I'm staying as calm as ever. I know than everything can happen in football and I believe in miracles. We've to enjoy ourselves on the pitch, but we're well aware we're in a difficult situation."
Mourinho's sentiments were echoed by defender Sergio Ramos, who stressed that Madrid are motivated despite the challenge that is ahead of them.
He added: "It's complicated for us, but if we want to be in the final we've got to defeat the best teams. We have to believe we can get back into it. 
"There is no bigger motivation than playing in a UEFA Champions League final. I've been here for eight years and never played in one. Nothing is more beautiful and we’re capable of doing the unthinkable."