However, the team has struggled since, with O'Neill presiding over a run of mediocre, labored displays that has left American owner Ellis Short to take the gamble of looking for an alternative with less than two months left of the season.
O'Neill was clearly not expecting Saturday's announcement, saying after the United loss: "I am as buoyant now as . I was nearly going to say as I have always been.
"What is it — the half-full or the half-empty glass? I'm even going to the three-quarters full."
Whoever does succeed O'Neill will be without Fletcher for the rest of the season because of injury, with the other main striker, Danny Graham, without a goal in seven matches since joining from Swansea in January.
Staying in the Premier League has never been so important in financial terms, with a lucrative, three-year broadcasting deal set to come into effect from the start of next season.
O'Neill is the fifth managerial departure in the league this season, after Di Matteo (Chelsea), Hughes (Queens Park Rangers), Nigel Adkins (Southampton) and Brian McDermott (Reading).