This isn't about ability. Quality. Though they'll say that if or when he is moved on in January. Indeed, you can argue it's not even about mentality. After all, it was Phillips who chose to dig in over the summer and insist he stay as management offered him a way out.
No, it's about that five per cent. The five per cent that separates good players from the great. The pretenders from the winners. And unfortunately, over these 18 months, Phillips hasn't shown that he has it.
But he did. He proved it. Under Marcelo Bielsa. Under the system the Argentine demanded of his Leeds players, Phillips thrived. He adjusted, and the local lad who'd come right through the club's academy system emerged as the outstanding performer of Bielsa-ball.
This is what attracted Guardiola. Such an admirer of Bielsa, of his football, City's manager was convinced they'd be signing a future world-beater. Just as he did with Rodri three years previous, Pep could see himself in Phillips. The Catalan would mould Phillips, develop him, just like with Rodri. Phillips would fulfil his potential. Guardiola would take him beyond it. As we say, it was the opportunity of a lifetime...
"We brought Kalvin here for his quality," said Guardiola at the start of this season.
"There are no twins in football, everyone has their own personality. Marcelo Bielsa gave Kalvin the best of Kalvin in his career at Leeds. I'd love to have done with Kalvin what Marcelo has done to him... (But) it is what it is."
That five per cent, Phillips couldn't find it. Perhaps he didn't want to. 'Fat-gate' - Guardiola calling out Phillips post-Qatar for returning to the Etihad Campus overweight - proved that. He'd let his manager down. He'd let the club down. Barely six months into this dream move and standards, all self-inflicted, had fallen away.
Guardiola, while publicly critical, also tried to help him: "He wants to change. It will maybe be a good lesson for him for the future. A footballer has to be perfect over 12 months - perfect. Even in holidays, he has to be perfect."
Problem was, Phillips didn't talk like he "wanted to change". Indeed, if you want to know why his City career is ending, you need only consider the midfielder's response as pre-season was getting underway.
"I just look at it and laugh," he said. "I'm the footballer and I'm living my dream, doing the thing I love. If someone wants to say something about me then that's fine.
"I'd gone to New York for three or four days after Qatar. I'd flown all day. I'd had breakfast, lunch, dinner and landed. Slept during the flight as well. And straight into football. I'd had a full day of eating. There is water retention and stuff on a flight. It was a misunderstanding really."
A misunderstanding? Pep didn't talk like that. Indeed, the player's reaction ("I'm enjoying myself") says everything you need to know. There was no mea culpa. No contrition. But this goes beyond failing to meet club standards. Beyond letting teammates down. It's about attitude. That intense, relentless attitude expected from City's pros. Phillips - today's Phillips - just doesn't fit.
For this column, we were looking for that circuit breaker. That move from Phillips to right these wrongs. News that he'd shunned the Treble parties to focus on his fitness. That he'd assembled a coaching team to prepare for the new preseason.
But we didn't get any of that. There was no going above and beyond. There was no desire to find that five per cent.
Florian Bluchel, Arsenal's Germany scout, recently spoke with Tribal Football about his time with Bayern Munich, and recalling how he spotted a teenage Rodri at Villarreal, said:
"He had everything, even at that age," Bluchel said. "It was just that physically, he was so slight."
Indeed before Villarreal, Rodri was cut loose by Rayo Majadahonda for being "so slight". The powerhouse we see today, the player his peers regard as the best defensive midfielder in the world - well, he was nothing like that in the early years of his pro career.
Even from the player who arrived from Atletico Madrid, Rodri has transformed himself physically. He's bought into everything his manager has demanded and is now reaping the rewards.
For the Rodri of four years ago, inside the club, they were saying the same of Phillips. All he had to do was buy in.
Eighteen months on and he's a Treble winner. He has money beyond his wildest dreams. So he can settle, can't he? He can "enjoy" himself. He was part of arguably the greatest team English football has seen.
Of course. And for most footballers, it's a decent argument. But Manchester City isn't built for most footballers, and City's manager doesn't do passengers.
It could've been all so different for Kalvin Phillips. All he had to do was follow his manager. Instead, he's just blown the opportunity of a lifetime.
