The midfielder played for the Republic of Ireland as a youth teamer.
And former Ireland coach O'Neill said, "I met Jack Grealish a number of years ago when we wanted him to play for the Republic of Ireland and he chose to play with England, no problem with that whatsoever – his father is English and he had played underage football for the Republic of Ireland, he made that decision and that's absolutely fine.
"He's very strong; he's much stronger than he was five years ago, a quality player, hard to rob."
O'Neill added: "You would hope that, at the end, Villa would have enough to pull themselves around and see it through. Of course, if you're depending on one player, which they have been over the past couple of months, that can cause a problem."