In 2013, Klopp moved to sign Mkhitaryan for £21.5m after the Armenian international had enjoyed a stellar spell with Shakhtar Donetsk and in three years at the Westfalenstadion, the playmaker netted 41 goals in 140 appearances in all competitions.
On Saturday, the duo will be on opposite sides as Liverpool host United at Anfield, but Mkhitaryan has not forgotten the impact that Klopp has had on his career.
The 28-year-old told German magazine 11 Freunde: "I had chosen Dortmund because I liked the way they play there. The players also liked me – and then there was the great coach, Jurgen Klopp. After I had talked to him, the decision had been made for me. I only wanted to go to Dortmund.
"When I came to Dortmund, Jurgen Klopp told me that I had to let go, and that it's not good to think about football all the time. I started to understand what he meant, and so slowly, this part of me has changed.
"I am thankful to Klopp. He worked on my personality and the psychological part. Klopp showed me the way. He supported me and told me I had to keep my head up because good things were coming. He helped me to become a better player."
Mkhitaryan has contributed one goal and five assists - the second-most in England's top flight - ahead of the showdown on Merseyside.