Kalvin Phillips transfer latest: Ex-Leeds United star backed to stay at Man City amid Newcastle United interest
He left having helped Leeds seal a long-awaited return to the Premier League, before playing a key role in the retention of top-flight status between 2020 and 2022.
However, it has not quite worked out for the 27-year-old at Manchester City and speculation regarding a January transfer window exit is rife.
Although Phillips has found opportunities limited under Pep Guardiola, Mills has insisted making the move was the right thing to do.
Speaking to The Yorkshire Post via Grosvenor Sport, Mills said: “It's a tough one for Kalvin, everybody would make that move. People have questioned if it was the right thing to do.
"Yes, it was, without a shadow of doubt.
"Nobody in their right mind would have turned that down, and that's irrelevant of the financial situation. It's a chance to go and prove yourself against the best, day in, day out. And that's what you do as a player, you want to ask yourself ‘am I good enough?'. So you back yourself. You believe in yourself.”
Phillips has made a total of 29 appearances in all competitions in his near-season and a half at City, 16 of which have been in the Premier League.
He was part of the club’s historic treble-winning squad last season but played the role of a supporting cast member, appearing mostly as a substitute.
Phillips is not the first player to have made a slow start to life at the Etihad Stadium.
Mills has referred to Rodri and Jack Grealish as examples of players who did not hit the ground running but are now among Guardiola’s most trusted lieutenants.
He said: "He's found it difficult as a lot of players have done when they've gone to Manchester City to work out how Manchester City play and to work out the Pep Guardiola way.
"That's not been easy for anybody going there, Jack Grealish, who was by far the best player at Aston Villa, struggled for a season or two.
"It's difficult. Even Rodri, who now is possibly the greatest midfield player in the world at the moment, had his challenges when he first arrived at the football club.”
Mills, however, does not think he will depart the Etihad Stadium in January.
Even with England’s Euro 20204 campaign looming and Phillips hoping to be a part of that, Mills said: "I don't think Kalvin will leave in January, I think he will give it until the end of the season to see if he can force his way in to get some game time.
"Then if he hasn't played at the end of this season, then he might have to make a decision on how much he actually wants to play first-team football.”