O'Neill saluted striker Fletcher after he left the treatment table to inspire his side to a precious victory over Reading.
He said: "Fletcher had done absolutely no work whatsoever. When you were talking to me yesterday, I genuinely wasn't sure that he could play in the game.
"He came out this morning just to do a bit of a loosener and thought he wanted to give it a go.
"I personally thought if he could see it to half-time and just slightly into the second half, he would do great for us.
"He has been terrific for us, really terrific. You just feel that he is likely to get you a goal in there.
"Danny Rose did very, very well, but when it came in, he turned it into a goal for us and it was great."