Black Cats coach firmly believes that the 23-year-old midfielder has the ability to continue his stunning form into the new season
“He has been sensational,” O'Neill told the Derry Journal.
“I can remember whenever he came on against Blackburn Rovers, he gave us that little bit of extra impetus and he never really looked back after that.
“Then whenever he got his chance in terms of starting he really took his chance and played well to ensure that he kept his place in the starting line-up.”
The former Celtic manager was keen to quash the notion that the winger would suffer from so-called 'second season syndrome', whereby a player who has impressed in their debut season suffers a dip in performance the following season.
“I know people are talking about the ‘second season syndrome’ but I have no doubt that he has the ability and the capacity to keep it going,” said O'Neill of his native Derry man.
“It has been an almost fairytale-like rise which he has had and even getting into the European Championship squad.
"And although he didn’t get much game time, he still needs to now get a break and come back a bit later than some of the rest of the squad and then take off from where he left off,” concluded O'Neill.