Guendouzi has been cast out of first-team training at the north London side after clear the air talks with his manager Mikel Arteta failed to reach a satisfactory conclusion.
Casoni has explained the problems he faced when he was in charge of him at Lorient between 2016 and 2017.
Speaking this week, he said: "Guendouzi's problem is not physical and is not technical. It's his attitude, it's not good for the team or the coach. My relationship with him wasn't very good.
"I picked him for a cup game against Nice but he got booked early on. The ref told me at half time to warn Guendouzi: one more foul and you're off, but in the second half nothing changed.
"I was left with no choice but to sub him off. When I did, he refused to shake my hand."
Casoni added: "He took his job seriously, his training was no problem, and his character is to always want to win.
"Sometimes when he talks it's good. But sometimes he talks badly. He talks too much.
"His talent isn't in question, that's not the issue. He can be a top, top player and I think he can still succeed abroad. It's up to him to change his attitude."