The defensive midfielder was signed by Newcastle United from Milan for £60 million in the summer window.
However, he was soon revealed to have been betting on football games, even ones involving himself.
Tonali was banned for ten months in October, with some accusing AC Milan of having known about his issues.
Pioli told reporters: “No I didn't know anything about Sandro's problem.
“He was polite, respected, and gave respect to others and was a sensible boy. Even if I think I have a close open relationship with a player I don't only talk about football. But what they do outside, I don't know everything about them."
“Well, at the moment they signed him it was a good deal, they signed a great deal and what happened after no one could have expected.
“Tonali, I am very close to him. When he was with us he was a positive lad always showing good will and application. He has fallen within something delicate. We are close to him and I hope this experience can be useful for him.
"Can he bounce back? Absolutely yes, with regards to his tactical and physical ability."
