But the England player quickly made an impression in creating the opening two goals for young striker Nathan Delfouneso and Carlos Cuellar, with John Carew netting the fourth from the penalty spot.
O'Neill said: "Ashley set up the opening two goals and he is a top-quality player who is so important to us.
"It is three years this month since he came to us from Watford and his contribution for us during that time has been immense."
Villa's second-choice goalkeeper Brad Guzan again highlighted his potential by saving a first-half penalty from Blackburn midfielder David Dunn, although he was at fault for Nikola Kalinic's goal for Rovers.
The American had earlier in the campaign kept out five spot-kicks in the Carling Cup tie at Sunderland - including four in the shoot-out.
O'Neill continued: "He has made another great save from the penalty and you are now almost expecting him to save penalties, which is rather unfair on him.
"He was disappointed with the goal they scored but we've got a Carling Cup semi-final with Blackburn on Tuesday and he is as big a reason as anyone why we are there.
"I'm delighted with his contribution but Brad Friedel is also playing excellently for us."
O'Neill made seven changes from the side beaten by Liverpool, and Delfouneso and another youngster in Fabien Delph created a favourable impression.
O'Neill said: "I'm delighted to have won the game.
"The players are desperate to play. We utilised the squad and the young lads, Nathan and Fabien, did very well for us."
O'Neill played down suggestions striker Emile Heskey will be allowed to leave Villa during January and revealed he hopes to add a couple of signings.
O'Neill said: "I don't know why Emile's name has been brought up but I wouldn't really want to sell players at this club.
"We are hoping to get a couple of players in because it is a long season ahead of us."
Blackburn boss Sam Allardyce was unhappy with the sending-off of midfielder El-Hadji Diouf for a first-half tackle on Villa defender Habib Beye.
He said: "I see a lot of those situations that never result in a sending-off.
"They will justify it by saying it was two-footed but the referee, Howard Webb, is a long way away.
"He did not know what sort of tackle it was because I have seen a playback on the video. If the assistant referee made the decision, then fine, but I don't think he did."
Allardyce also made nine changes and praised his side for their second-half comeback after a poor opening 45 minutes.
He said: "I have to give them great credit for the way they played in the second half but all the damage was done in the first half for us.
"We gave ourselves such an uphill battle, the first half was a disaster for us. We went 2-0 down, missed a penalty and had a player sent off.
"At least we showed the right professional approach in the second half to get back into the game, even with 10 men.
"The Carling Cup is our priority and I made a lot of changes but I thought young players like Phil Jones and Martin Olsson never looked out of place."