The Scotland international was not even expected to be involved as he continued his recovery from the ankle injury which had sidelined him for the previous two games.
However, he declared himself fit this morning to hand his manager and teammates a boost, and then capped an impressive display with the second of three unanswered goals to help the Black Cats climb out of the English Premier League relegation zone.
O'Neill 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."
A year to the day since making his senior debut in O'Neill's first game in charge, James McClean opened the scoring for Sunderland with just three minutes on the clock.
Reading goalkeeper Adam Federici could only punch Adam Johnson's inswinging corner to McClean on the edge of the box, allowing the Irish international to skid it across the surface into the net.
Fletcher doubled the lead with an audacious back-heel just before the half-hour mark, flicking Danny Rose's low cross back past Federici.
Reading struggled to get back into the game as Sunderland looked to press their advantage, eventually making it three in the closing minutes of the game with Stephane Sessegnon slotting home a long ball from Carlos Cuellar.
O'Neill, who received the tacit support of Sunderland 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."
Opposite number Brian McDermott admitted he is now deep in a battle to avoid relegation and preserve his own job.
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."
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