The pair worked together at Borussia Dortmund and will meet again for their Champions League round 16 tie.
“He's a coach who has been crucial to my life as a footballer. I have to be thankful to him for where I am in my life right now. He's just great - as a person and as a coach," Lewandowski said.
“We've talked so much about football and life, and I've learned a lot from him. Tactically, there are a few things. But what he has given me above all is the belief that I can play at the highest level.
“He has awakened this idea in me and has always shown me the next step forward. I didn't know that I had such great potential."