The Pole was signed by Klopp in 2010 whilst he was in charge of Borussia Dortmund.
Lewandowski has since gone on to become one of the world's best strikers, and he has credited Klopp with significantly helping his development.
He told the Players' Tribune: "Jurgen was not only a father figure to me. As a coach, he was like the 'bad' teacher. And I mean that in the best sense of the word.
"Let me explain. Think back to you when you were in school. Which teacher do you remember the most? Not the one who made life easy for you and never expected anything from you.
"No, no, no. You remember the bad teacher, the one who was strict with you. The one who put pressure on you and did everything to get the best out of you.
"That's the teacher who made you better, right? And Jurgen was like that."
The 32-year-old added: "I could talk to Jurgen about anything. I could trust him. He is a family man, and he has so much empathy for what goes on in your private life."