The 26-year-old will observe the remainder of the club's tormented season from afar after being told to stay away from the training ground after being sent off for a rash challenge during Sunday's damaging 3-0 Barclays Premier League defeat at Liverpool.
Moncur, the last Newcastle captain to lift a trophy, said: "I'm just disappointed because last season, he started to look a decent player. The sad thing is, he will be hard to change."
He added: "Big Sam (Allardyce) signed him and at the time, he thought he could do something with Joey Barton.
"But often when a manager thinks he can do that, it proves not to be the case."
Barton was sent off for a two-footed lunge on Xabi Alonso which saw him banned for the last three games of the campaign, and is then understood to have become embroiled in a furious row with manager Alan Shearer in the dressing room afterwards.
The club on Tuesday suspended him until further notice to write a new chapter in the troubled player's chequered career. His appearance at Anfield was his first since he fractured a metatarsal in January and could be his last - with the club's hierarchy now determined to get him off the books whether Shearer remains in charge or not.
Former Magpies defender John Anderson has warned the managerial situation and the continuing uncertainty at St James' Park could make that easier said than done.
He said: "You have got to be very careful with that. It all depends on who is in charge here next season.
"Yes, if Alan is here next season, that's his last game. But you never know, another manager might quite fancy him as a player. To be fair, he's a very, very lucky boy anyway. He has let himself down, he's let everybody down."