"I must admit I would have given him no chance of playing in the FA Cup game at Crystal Palace last weekend," said the Northern Irishman.
"He came off against Manchester United and came in the next day for treatment.
"He was very sore and hobbling. I really could not have seen him play the FA Cup game against Palace but he did and he scored a fine equalising goal.
"It was a genuine injury and he did remarkably well to recover and play at Palace."
O'Neill is impressed with the impact Petrov has made in his two years at the club, not only on the pitch but off it.
He said: "In the last two seasons he has played here at Villa he has been absolutely fantastic.
"He is very modest about his game. He always says that he has to play so well to make up for lost time when he did not play so well when he first came to Villa from Celtic.
"But his last two seasons have been so consistently excellent. I think he has taken on the mantle of captaincy not just on the field but also off the field.
"He tries to get things organised for the players and he takes care about certain things.
"He is also incredibly generous as well with some of the things he has done for other players.
"He wouldn't mention that and I think the other players have a healthy respect for him."
O'Neill believes a match at Pride Park could have been significant in Petrov's development.
He added: "The turning point to his career might well have been the back end of the season before last when he scored a really good goal against Derby.
"Basically, he was coming in to play a more central role in midfield.
"He had been previously playing on the right hand side when maybe when we only had one wide player at the time.
"He was coming in as a third midfield player having to break wide and not being in a central position.
"I think his form now is mainly down to his own confidence which he had to regain and which he has done splendidly."
Petrov has also had to win over the Villa fans after the slow start to his career in the Midlands when O'Neill signed him from Celtic in the summer of 2006.
He has now become a crowd favourite and O'Neill would love to see Petrov reward the Villa fans for the backing they have given the team by lifting the Carling Cup next weekend when Villa face Manchester United in the final.
O'Neill added: "It would be difficult for me to demand certain things from the crowds.
"Whether the crowd gets impatient with us they still turn up in their droves to support us and you cannot ask any more in this day and age when money is hard to find.
"From that point of view it would be unfair of me to start asking fans to do something else.
"Their response has been through the turnstiles. If there are certain things that irritate them through the course of the game, well so be it.
"This is the nature of the game but you have to treat this on its merit with tickets for Wembley being very expensive. Even the cheapest are very expensive.
"I am hoping that when Wembley comes around the fans will have a really fine day.
"It will not be what we are going down for. We are going to Wembley to try and win the Cup."