Keegan told a news conference at St James' Park: "I know Joey, I had him at Manchester City since he was 16 or 17 and I used to watch him in the juniors.
"I can't control what has gone on before or what will happen in a courtroom.
"What I told Joey on Thursday is that as a Newcastle United player I will support him 100% and help in any way I can and he has said openly that he needs help.
"The best people to help him are the people that know him and we know him.
"He's an excellent player, he can improve, and he's had some problems.
"I told him he has my full support, I've given it to him before and he has let me down but he's got it again because having spoken to him I am convinced all he wants to do now is play for this football club."