Barton has been handed a three-match ban by the Football Association after lashing out at Blackburn's Morten Gamst Pedersen during a fixture at St James' Park on Wednesday.
The Magpies midfielder reacted angrily to being brushed on the shoulder by the Rovers winger, punching him in the chest.
It is not the first time Barton has been in trouble with the authorities, and he admits he faces a constant battle to control his anger issues.
Mills, though, insists his former City team-mate is working hard to alter his ways and honestly believes that he regrets his latest act of petulance.
"I spoke to Joey yesterday (Thursday) and he regrets what he did," Mills told Sky Sports News Radio.
"From his point of view it was a stupid reaction. There had been a bit of a couple of off the ball things with Pedersen earlier in the game and he just shoulder-barged him, not particularly hard, and Joey reacted.
"Joey knows he reacted badly and shouldn't have done it and it's a shame because Joey's form, last season and at the start of this season, has been fantastic.
"He has been integral to what Newcastle are doing, and their position in the league, and I think we have to be careful. He has made a mistake, but it wasn't the worst punch in the world.
"Morten Gamst Pedersen went down slightly too easy, I think he could have taken a standing count of eight quite easily but he crumpled to the floor like it was a knock-out punch. It wasn't a swinging haymaker.
"You can't justify what Joey did, but I think we have to get it in perspective. Had it been any other player, it would have been a three-game ban and we'd have forgotten about it.
"The fact that it's Joey and his history, and he does have this constant battle, he's been in prison, he's had a lot to deal with. I think Joey realises that he made a mistake, he will put this out of his mind and it will now be all about how he reacts when he comes back."