The 23-year-old has quickly won over the Black Cats supporters with performances that also led to his inclusion in the Republic of Ireland's Euro 2012 squad.
O'Neill said: "It is a big season for him. There is obviously always talk about second season syndrome, that type of stuff, but I have a good belief that I don't think it will affect him.
"He has come back, he's done exceptionally well in training here. He looks every bit as strong as he was before and he looks every bit as determined.
"I don't think those sort of things will bother him."