Buvac tendered his resignation on Sunday after a clash with Klopp, it has been claimed.
Heidel worked with both men at Mainz and confirmed Buvac has returned to the city.
He told BILD: "He is currently in Mainz. I never imagined that they would not work together again. I only know them together. But I also know that they are two highly emotional guys. Whenever it started in Mainz, you thought the door would fly out, and probably only one will come out. Instead, they came out arm in arm and were down to the pub.
"Buvac was, before I brought him as an assistant coach to Kloppo and Mainz, once head coach, namely at SC Neukirchen. If you only do one role for 20 years, namely assistant coach, that's another challenge as a head coach. It was not his ambition in the past, because he was never the guy in the front line. There was always the sportsman Klopp, while Buvac 'The Brain' was the brain. The two fit together extremely well, also in human terms.
"Now he will probably think about whether he dare to take this step. But even with another coach it would be difficult for him."