The £4million England left-back, 27, has lost his place as O'Neill plays right-back Luke Young out of position instead.
But O'Neill said of the former Reading star: "Nicky's got plenty of determination. I'm sure he'll come through this.
"There will be hiccups but Nicky will come again. He's trying to get used to the environment and what I'm asking him to do - but he's a lovely footballer."