The Poland international scored a brilliant solo effort at the HDI-Arena, with his boss in awe of the way in which the striker made the most of such a small opening
The Poland international scored his side's second of the afternoon by weaving his way through the opposition defence before slotting the ball past Ron-Robert Zieler.
Klopp was left stunned by the ingenuity of the strike and was delighted with his side's showing, despite the absence of a number of first-team players through injury.
"With Lewandowski's goal, I couldn't see anyone he could pass to," the 46-year-old BVB boss told the club's official website.
"Then he went into the gap and he used it brilliantly. It was a world class goal.
"It was a very tough game, though. None of the 22 players walked on the pich and found it easy. The two teams had to work very hard.
"We had to do without some key players and had to rest some, but Hannover also were missing many important players.
"We were able to control the first half without shining and get the first goal from a set-piece. I was very happy because we've been working on that.
"With the second goal, we turned the game in the right direction and from then on it was very difficult for the opponent.
"I'd have loved it if we had been able to get a third goal on the break sooner because if it had been 2-1, the crowd would have been right behind Hannover and we know what can happen here from personal experience.
"My team approched the game very seriously."