The 23-year old winger rose to prominence last season for the Black Cats, winning a spot in the Republic of Ireland Euro2012 squad as a result, and O'Neill feels he can keep that form going as he gets stronger and better conditioned for Premier League football.
"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," O'Neill said.
"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."