The pair worked together at Mainz and BVB before Klopp's departure for Anfield.
"He saw my potential when I was still skeptical and gave me the opportunity and the necessary advice to become the player I am today," said Subotic.
"He has a significant share in my career. While I walked the steps and had propped the weights, he stood by my side, to explain how and why.
"As important as Kloppo is for me, even in 50 years we will be in contact. Many players who have worked with him will say probably the same."