After his latest all-night drink-fuelled binge, Aston Villa manager Roberto Di Matteo has warned Grealish to "grow up quickly".
O'Neill lost the midfielder to England a year ago, but says: "I certainly would have been able to sort him out at club level because I'd have been working with him every day. He wouldn't have had the same problems.
"If I was his club manager I don't think that he would have been involved in the shenanigans he has been, I promise you."
On Grealish's England commitment, O'Neill added: "Whether young Grealish was coerced into doing something, I don't know. Once he had decided to go for England, that is fine, he has made his decision. He has to live with that and I move on."