O'Neill said: "If you had said to me that within one year Ashley would have played for the full England team, then I would have been surprised.
"But I'm delighted with him and all the things he is showing; I always thought he was capable of producing.
"It is getting to the stage where people are definitely keeping an eye on him, definitely double-covering him.
"People are making changes to combat him. Sunderland changed their side around last weekend to accommodate him.
"That is great credit to Ashley. But it is something we have to try and utilise to our advantage, with the extra space that may become available elsewhere."