The Portuguese feels his side lacked the physical strength to deal with the Poland striker, while adding that Tuesday's game is the "most important match in 10 years" for the club
The striker scored all four of BVB's goals in a stunning 4-1 win at Signal Iduna Park last Wednesday, and looking ahead to Tuesday's second leg, the Portuguese coach admitted his players lacked a physical edge to deal with the Poland international.
"In the first leg, we didn't even foul Lewandowski once. We were way too naive and pure," the 50-year-old told a press conference on Monday.
"We have to play it minute by minute and goal by goal until the last breath of the match."
Madrid will need to produce an incredible comeback if they are to advance to the final at Wembley, but Mourinho remains confident his side have what it takes to do just that.
"It's only football and everything is possible. We have to enjoy ourselves on the pitch, although we are in a difficult situation," he added.
"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."