And Mills says a story involving Leeds United and Middlesbrough helped him see McClaren's true colours.
Indeed, the defender refused to play against Leeds while on loan at Boro and that had upset the hierarchy at the Riverside Stadium. But McClaren failed to be honest with him and pretended he would be signing for the Yorkshire club.
Mills is quoted as telling talkSPORT by the Chronicle: "A brilliant coach. Man-management; well, not so good.
"Of course when I was at Middlesbrough I should have signed there, the deal fell through last minute.
"I sort of walked out, and refused to play - I was on loan from Leeds at the time and I didn’t want to play against my parent club when you could do that back in the day.
"He should have just been honest and said: ‘Look, you’ve upset the chairman, you’ve upset the chief exec for what you did in January. I’d like to sign you but they don’t.’"
McClaren's man management has certainly come under scrutiny at Newcastle this season and the Mills example is consistent with what we've seen at the Magpies.
It seems the Newcastle boss just doesn't have the courage to tell it like it is. As with Mills, so often this season all McClaren has had to do is stand up to his players and admit how bad they've been. But, as with Mills, he's shied away from being honest and doing the right thing.
Mills spent five years at Leeds and, after failing to join Middlesbrough, instead moved to Manchester City, where he spent another five years.