Lewandowski has spent much of the season on the bench, but Klopp says he deserved his chance and his goal which left his side needing just five wins from their last nine matches to mathematically secure the title.
"He is a great lad," said Klopp. "I am so glad for him that we have won a game 1-0 and the biggest contribution to the win was made by him.
"We have been training intensively with him in a new position for two weeks now and you could see that today."
The Polish striker admitted that the goal had done him good, but like everybody else at the Westphalian club, title talk remains taboo.
"I am delighted with my goal and the win," he said. "We played really well and deserved to win.
"Of course the goal was very important for me personally because it is quite a long time since I last scored.
"But there is still a long way to go this season and we don't want to talk about the title yet."
Cologne coach Frank Schafer, who saw his captain and star striker Lukas Podolski limp off to compound his woes, said his side had been unlucky not to get a point.
"We were well organised in the first half and it is even more frustrating that we went behind to this silly goal," he said.
"It would have been important for us to get to half time at 0-0.
"Only a lucky punch would have got us a point in the second half and we had a chance, but it was just not meant to be."