Sacked by Spurs last Friday, Postecoglou sent a Whatsapp text to Bissouma as part of his farewell to the players.
The Australian wrote: "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."
Thank-you gaffer
Bissouma replied: "Thank you gaffer and thank you really much for everything on and off the pitch I'll always be grateful.
"The way you believed me and trusted me was incredible. I'm gonna miss you but it's football life.
"Got a lot to be said but 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."
The exchange ended with the 59-year-old responding: "Always by your side Biss ????."