The Scotland international has made a quick recovery from an ankle problem and helped lift Sunderland out of the bottom three on the back of only their second home win of the season.
Fletcher scored the second goal of the game with an audacious flick from Danny Rose's cross in the 28th minute.
O'Neill said on Tuesday night: "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.
"I didn't want people starting to get carried away with it.
"I just want to try with all my might to put a team together here that not only can compete, but that a team that people would look forward to seeing play."