The 25-year-old England winger has found the going tough on Wearside since announcing his arrival from Manchester City in a thrilling debut in a 2-0 Capital One Cup victory over Morecambe on August 28.
O'Neill said: "First of all when he arrived here, his opening game against Morecambe was fantastic, he played really well in the game.
"Then we travelled down to Swansea and he couldn't really get into it, and since that he went with England, got injured, came back with us, missed games when he was hoping to be fit and his thigh injury was still niggling, and there's nothing more frustrating and more annoying if you are a person who has not been injured that often.
"That has had some sort of an effect. Once he gets a run, he will be fine.
"Coming here and maybe feeling that everybody is expecting him just to win games on his own - it's a thought.
"It's not necessarily what I actually believe, but it's a thought, and perhaps that's something that he should concern himself less with."