Walker ended an nine-year stay at Tottenham in pre-season, as the England right-back secured a transfer to City which could eventually be worth £50 million.
Although many were quick to praise Spurs for getting a good deal, particularly with Kieran Trippier emerging as viable competition for Walker, Pochettino was not happy with how the move came about.
Walker and Trippier found themselves in a good tussle for the right-back position towards the end of last season and often rotated, but after Spurs' 4-0 win over Watford on April 8, Walker told Pochettino of his intention to leave the club.
And Pochettino was left baffled that the 27-year-old decided to reveal his desire in the midst of Spurs' title challenge.
Writing in the Daily Mail's serialisation of the book Brave New World, Pochettino said: "Walker came to my office after the Watford game.
"'Gaffer, I've been at Tottenham for nine years. I've thought about it and my heart isn't here any more. Nor is my head. I've given all I have to give. I wanted to tell you before I tell my agent that I want to leave this summer.'
"'Kyle, you have to stay professional. There's a month and a half to go. We're battling for the Premier League and FA Cup. We have to be focused and finish the campaign strongly.'
"'Okay, gaffer. But it's already decided.'
"'Well, that doesn't just depend on you or me. It depends on the club, above all. You've disappointed me because you've decided to tell me that you want to leave when there is a month and bit left in the competition.
"'You could've sucked it up, kept quiet, trained, played and helped the team when not picked, and at the end of the season you could have come and told me.'
"Miguel [D'Agostino, first-team coach] was present. I always try to ensure there are witnesses during private conversations. I considered it to be an alarming lack of respect for his team-mates and a slap in the face for the club. None of that could be explained to the public at the time.
"I had to bite my tongue. From that point, the rumours started to proliferate."