Newcastle boss Kevin Keegan says he has seen "a different Joey Barton" after the midfielder's spell in prison.
Keegan managed Barton at Manchester City
Barton spent a week in a Liverpool prison over New Year after being charged with assault and affray following an incident on Boxing night.
Keegan told BBC One's Football Focus: "I honestly think prison has been a reality check for him.
"I'm watching him every minute of every day, like all my players, and I've got to say I see a different Joey Barton."
Barton made his first appearance in a Newcastle shirt since the incident when he came on as a second-half substitute against Arsenal on Wednesday.
Barton played his first game since the incident on Wednesday
Keegan said he saw evidence of Barton's changed character during that cameo appearance.
"There were a couple of times he might have done something silly, but his brain took over and he thought 'no I'm not going to do that any more'," he said.
"You've got to give him another chance. Everyone deserves that, even someone who maybe has let you down a couple of times, like he has me.
"I've said 'clean piece of paper, don't let me down, I'll support you 100%'. And I will."
The Newcastle boss knows Barton better than most, having managed the central midfielder when he was coming through the ranks at Manchester City.
"I know Joey as well as he knows himself, having seen him come through as a 16 or 17-year-old," Keegan said.
"Joey had to scrap and fight like you wouldn't believe. He was always just a little bit behind everyone else.
"Some lads were getting £200 a week, Joey was getting £150. He'd get a one-year contract, while they would get two.
"He's had to fight that all the way and that sort of desperation helped him get to a level a lot of people thought he wouldn't achieve.
"When he got there, he struggled to handle that for different reasons."
Don't be fooled by this transfer window - the funds are definitely there for us
Meanwhile, Keegan said Newcastle fans should not be worried about the club's lack of activity in the January transfer window.
The club failed to bring in a single new player during the window.
"Don't be fooled by this transfer window - the funds are definitely there for us," Keegan said.
"That was my choice not to bring anyone in. We could have brought in four or five players and spent some money.
"But I didn't want to do that for the sake of it. In the summer we'll bring whatever we feel is needed for the next jump.
"That next jump is to be fighting for a European place. Mike Ashley will support me, no doubt about that. He loves this club and is excited and enthusiastic as I am."
Watch the full interview on Football Focus on BBC1 on Saturday 2 February from 1210 GMT.