Despite bringing Europa League glory to Spurs, Postecoglou was relieved of his duties last Friday.
Bissouma has since revealed exactly what was said between him and his now former manager.
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Indeed, the 28-year-old shared a screenshot of a series of WhatsApp messages that were exchanged.
Postecoglou said: "Hi Biss. Sorry I missed your call. I am very proud of you. It was an honour to share a dressing room with you and appreciate how much you believed in what we were trying to achieve. I wish you only the best for the future. I will always be following. Much love from my family to yours."
Bissouma replied: "Thank you, gaffer, and thank you really much for everything you did for me on and off the pitch. I'll always be grateful. The way you believed [in] me and trusted me was incredible. I'm gonna miss you, but it's football life... I think you know what I'm thinking about you already, so I'm just wishing you all the best for the future and may God protect you and your family #bigboss love."
Postecoglou then stated: "Always by your side Biss."
It remains to be seen where Postecoglou may end up next.
Brentford's Thomas Frank is reportedly being lined up as his successor at Spurs.