But O'Neill's "kick up the backside" worked wonders and Villa responded with four goals in 13 minutes from Downing (two), Emile Heskey and Gabriel Agbonlahor in running out 5-2 winners.
It kept Villa on course for a Champions League spot via a top-four finish in the Premier League as they put their recent goalscoring problems - just three in the previous seven league games - behind them.
Downing said: "The manager wasn't very happy at half-time. It took a gee-up from him and a few other players to get us going because it wasn't acceptable really.
"It worked wonders because in the second half we looked a different team and played some great football.
"The manager is a great man motivator and half-time was the perfect example with a kick up the backside.
"He demanded more of us as players and there are some big characters in the dressing room and a few things were said and it worked.
"It was a slow game for a long time and once we got the second goal, it gave us a bit of confidence and we played some great stuff in a 15-minute spell and killed the game off."
Downing missed the first half of the season after his £12million move from Middlesbrough in the summer after breaking his foot in April, in a challenge with Villa skipper Stiliyan Petrov.
But he has quickly made a big impression with O'Neill.
He said: "I think Stewart has done wonderfully well for us.
"He's had no pre-season, he came straight into the side from injury and has been excellent.
"He did some wonderful things in the game against Burnley. I think he is just getting really properly fit.
"Sometimes the crowd get a bit anxious when he goes 10 yards and then checks back.
"But the first goal he stuck in brilliantly and you can immediately see a lift of confidence - and his second goal came because the confidence was up."
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