Speaking to Eurosport Germany’s Vocal Athletes podcast, the six-time Grand Slam champion revealed: "My promise to him was that I had to tell him open and honestly the truth, because everything else doesn't work and I can't embellish anything.
The reason why you might not be so good anymore is because you felt too good as number one, you didn't improve. The other players like Nadal and Federer have adjusted their game. I don't like your positioning on the court, I don't like your serve at all.’
The pair's collaboration ended in 2016 when the Serbian took a break from the court and had an operation on an elbow injury - but Becker says they still have a good relationship.
"At the subsequent Grand Slams we always met whenever he had a question about a player," he said. "My door was always open. A player-coach partnership like that is above all characterised by trust. I can only help a player if he openly addresses his worries and fears. He has to be totally honest and I really appreciate that trust."
And Becker refused to rule out the possibility of going back into coaching - but only if he was no longer in his current role of head of men's tennis in Germany.
"If there are players who interest me, who allow me to coach so and want to be open with me, then that's something I could definitely consider," he added. "The problem is that it is very time-consuming... I have a family and I have other professional projects that I need to manage.
"But I don't want to rule it out completely. Tennis is my great love."