The Scotland international was not even expected to be involved as he continued his recovery from an ankle injury which had sidelined him for the previous two games.
However, he declared himself fit this morning to hand his manager and team-mates a boost, and then capped an impressive display with the second of three unanswered goals to help the Black Cats climb out of the Barclays Premier League relegation zone.
O'Neill said: "Fletcher had done absolutely no work whatsoever. When you were talking to me, 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."
If it was Fletcher's 28th-minute strike, his seventh of the season which came courtesy of an audacious back-heel from Rose's low cross, which gave Sunderland breathing space, it was James McClean's third-minute opener which lifted the tension hanging over the Stadium of Light.
A year to the day since being handed his senior debut for the club in O'Neill's first game at the helm, the winger thumped home a low shot from the edge of the box after Adam Federici had punched Adam Johnson's corner to him.
Reading, who have won only one league game since their promotion from the npower Championship, never looked like getting back into it, although it was not until injury time that Stephane Sessegnon ran on to Carlos Cuellar's long ball and rounded Federici to cement just a third league win in 24 attempts.
O'Neill, who received the tacit support of owner and chairman Ellis Short before kick-off, was delighted to have collected the three points, but warned it was only a start.
He said: "I just feel that this season will be really tough for us, I accept that. I kind of thought that anyway at the start of the year.
"I didn't want people starting to get carried away with it. From my own viewpoint, I just want to try with all my might to put a team together here that not only can we compete, but that a team that people would look forward to seeing play.
"But that's sometimes easier said than done."
But while there was relief for O'Neill, opposite number Brian McDermott headed away from Wearside knowing he is in the thick of a survival battle.
He said: "Do I fear for my future? I don't have fear about it. I have been in football for 30-odd years now and I know this club is in a far healthier position than it was a year ago, two years ago when we were starting League One in the face, three years ago.
"Where we are now as a club, we are in a far healthier position, so that's the most important thing for me, the club.
"I am just disappointed with the results. I have been used to winning games over a long period of time and we are not winning games."