Petrov was an attack-minded player at Parkhead and he played a major role under O'Neill, including helping the Bhoys reach the UEFA Cup final five years ago.

He became O'Neill's first major signing at Aston Villa with an £8million switch in August 2006 but only now is he living up to that price tag.

The Bulgaria international has been converted into a defensive player giving protection to the back four and produced a man-of-the-match performance in helping Villa defeat Sunderland 2-1 to climb into third place in the Premier League.

O'Neill said: "Petrov was definitely inconsistent before. He had never really captured the sort of form that I had seen him display at Celtic - and I mean Celtic on European nights where he was fantastic.

"He has had to adapt. The role has changed for him here. At Celtic he was basically a midfield player getting beyond the centre-forwards - and I think I could score goals if I had Chris Sutton and Henrik Larsson playing up front when coming through from midfield!

"But here he is playing the holding role in the side. He is getting plenty of touches early on in games, his confidence is high, his strength is terrific.

"It is the sort of strength in holding people off that I've known Stiliyan for for five years at Celtic. This is what I've been pleased with and, when he gets up to head the ball, he is brave.

"Confidence is an amazing thing in football. His confidence was low last season and then it looked as if he was deliberating as to whether to go and challenge someone.

"Now confidence is so high, he is going in, winning the balls, he is setting things in motion and he can play himself. I really could not be more pleased with him. I thought he was outstanding against Sunderland."

Defender Curtis Davies echoed O'Neill's sentiments about Petrov who has won the fans over with his displays during the current campaign.

He said: "The man of the match at Villa normally goes to about one of four players. You all know who those players are. For Stan (Petrov) to get it was brilliant and I think he was top class again against Sunderland.

"He is showing why he is staying in that central midfield and it is going to be hard to displace him.

"I wasn't here when he first came here but, since I've been here, Stan has been spot-on and you can't really fault him.

"Last year he had a stage where he wasn't involved for six or seven games but he is a strong character, a strong player and he can always come back and put in performances like that."