Lewandowski left Bayern last year for Barcelona.
"There are some signings that were decisive for the club. One of them was Robert Lewandowski. He is a goal machine and a really good guy. I have never seen a more honest player than him," he told Kicker.
"I could talk to him about anything. He always gave me an honest answer and not one that was prefabricated or in his favour. You must understand that this is not entirely normal as my job was to promote the best interests of the club and not just his best interests."
Rummenigge says the Pole was a great help, not only on the field.
"In the end, Robert was an incredibly important adviser to me. You can discuss football with him at an extremely high level and he is also a relaxed guy - extremely smart," he added.