Nani saw red before the hour mark with the score at 1-1, but Villa could not make the advantage stick and missed out on being a club who did a season’s double over the Champions.
But O'Neill was not critical of his players and appeared happy with the point.
"The game was balanced at that time at one each, I thought that there was an opportunity of course," he told Sky Sports.
"You always think that if you've got an extra man for that length of time that you can be able to make the best use of it.
"But Manchester United are not Manchester United for nothing.
"They're a quality side, they've had problems before and they've been able to see them through.
"I thought they demonstrated at times how brilliant they can be in terms of possession of the ball. They've got very, very good players playing for them.
"But I think we're improving. Tonight I thought that we probably just didn't do enough to warrant a win.
"We didn't create enough perhaps in the second half of the game and we may have got a little bit tentative.
"I don't really know the reason why but we've got some really fine players playing at the football club and I've been delighted with them for quite some considerable time."