Following a successful seven-year spell at boyhood club Leeds United, Phillips decided to take the next step in his career by joining Man City for a reported £42m in July 2022, with the hope of establishing himself as a first-team regular at one of the best teams in Europe.
However, the 28-year-old was reduced to just four starts and six substitute outings in his first season at the club - City's famous treble-winning campaign - largely due to injuries and the exceptional form of midfield lynchpin Rodri.
Guardiola has since opted against selecting Phillips this term, with summer signings Mateo Kovacic and Matheus Nunes, as well as defensive trio John Stones, Manuel Akanji and Rico Lewis, all preferred in a midfield pivot role either alongside Rodri or acting as cover for the Spaniard when he has been suspended.
Phillips has started only five of his 31 appearances for Man City across all competitions since joining the club 18 months ago, and after Guardiola admitted at the end of last year that he "struggles to see" the Yorkshireman in his first-team plans, a loan move to West Ham United has since transpired.
The England international was unveiled as a West Ham player on Friday morning and he will spend the remainder of the season on loan at the London Stadium, with the Hammers believed to have the option to buy the midfielder for around £40m in the summer.
Speaking to Sports Mole following the release of a brand new BBC Sounds podcast 'Planet Premier League' for which he is joined by Cesc Fabregas, Onuoha shared his views on why Phillips's spell at Man City has not worked out.
"I think from what you hear, it's not a personal thing and that's a really positive sign because it shows that he was trying, he was engaged, people liked him," said Onuoha.
"But for the way that City play - and the same can be said for last season when things changed with [Joao] Cancelo, who all of a sudden wasn't essential anymore, when [Aymeric] Laporte wasn't essential anymore, people like this - the way that Pep and City play is constantly evolving and sometimes you need a certain style of player in certain positions.
"This has sort of led to the tremendous rise of Rodri, and if we put Rodri and Kalvin Phillips next to each other - whether someone wants to say they are good players, bad players, whatever - they are very different.
"You wouldn't want to necessarily change the way that the team plays based around who's playing the position. Rodri you can see has got a link to Fernandinho in terms of how those two [have played at Man City]. Fernandinho would be getting around the field, going up higher, linking the game from wherever, would take the ball under pressure, he really understood the game, and that feels like Rodri, but then it doesn't feel like Kalvin.
"Kalvin's range of passing is probably a bit more expansive, the way he wants to get around the field is a bit different, the places where he takes the ball is a bit different, and so with Pep, I'm sure he wanted to give him game time, but he knew that if he did the game would look very different.
"Unfortunately, it didn't work out and I'm sure Kalvin was hugely disappointed about his time, but then in the same breath, he's probably not because he won a treble last season, he's played in the Champions League and he's going to a West Ham side who are up in sixth (in the Premier League) anyway.
"The loss itself is probably not as big as some people perceive from the outside, and he's had the chance to gain some valuable experience being in one of the most significant seasons (in Man City's history) and group of players that's ever existed in English football, based on what happened last season.
"It didn't work out, Pep obviously just didn't see a vision for him being in that team, but Kalvin didn't take it personally and I'm sure Pep has explained it to him. Thankfully, now he's got the chance to go to West Ham and hopefully play some games and play some good football."
Onuoha believes that Phillips, now at West Ham, could regularly link arms with summer signings Edson Alvarez and James Ward-Prowse in a "competitive" three-man midfield for David Moyes, which could, though, reduce Tomas Soucek's game time.
"I was wondering who Kalvin was going to play instead of, but I think the midfield three as I see it, and I could be very wrong, you can now see Alvarez, Phillips and Ward-Prowse. I think that would be the three.
"That three would then [play behind Lucas] Paqueta, [Mohammed] Kudus, and [Jarrod] Bowen, that's a good top six that. I like that a lot. Obviously I could be wrong and David Moyes could have a different vision altogether, but I think that's a very competitive midfield that's linking a very good attack and that's got a solid backline behind it."
"I think it's a good move (for Phillips)," Onuoha added. "I think it's a great opportunity for Kalvin to go to another experienced manager, going into a team that's progressive, still in Europe as well, so Kalvin could win another European trophy. I'm there for it, good luck to him."
Asked whether Man City may look to sign a replacement of a similar profile to Phillips should he decide to leave the club permanently in the summer, Onuoha said: "Yeah probably, although it depends how they play, because sometimes when you've got the centre-backs coming into midfield anyway, all of a sudden it feels different.
"I think it's not always the case of one in one out in football, because unfortunately for Kalvin when he was in the squad, he was always on the bench, and very rarely was he ever being considered beyond the last few minutes of a game where the game is done and time is set.
"Do you really need to spend money in January to bring someone in that has that same sort of role? Probably not, but I'm sure long term, City - as is the case with other teams with quite progressive transfer policies - they would have already been thinking about who they wanted in for certain positions from a long time ago, as opposed to being reactive to when somebody leaves, just trying to panic to get somebody back in."
One midfielder who has been linked with a possible move to the Etihad is Bayern Munich's Joshua Kimmich, who has been tipped to leave the perennial Bundesliga winners having entered the final 18 months of his contract.
Recent reports have suggested that Man City are prepared to wait until the summer to step up their pursuit of Kimmich, but the English, European and world champions could face competition from Premier League rivals including Liverpool and Manchester United.