Heidel gave both Klopp and Tuchel their big breaks.
Though Tuchel was not Klopp's immediate successor at Mainz, only a year separated the end of the Liverpool manager's reign there and the start of Tuchel's ascent through the hierarchies of European football.
"I was saying that we, as a club, had to be a bit more engaging with our public," recalled Heidel in a biography about Tuchel.
"Then a phrase slipped out and I said: 'When Kloppo was here…'
"At that, Thomas let rip and shouted at me. What was I thinking? The conversation was over in that moment. And I thought, 'Oops! I've obviously hit on a sore point'. He was really upset. That's where the rivalry with Klopp stems from."
Tuchel, 47, was more successful at Mainz than Klopp, qualifying for the Europa League by finishing fifth in the Bundesliga and never lower than 13th. He then followed Klopp at Borussia Dortmund, where he couldn't match his predecessor's two Bundesliga titles and German Cup win, though he finished second and third and won the German Cup.