But the Bulgaria international struggled to adjust to the Premier League and became the target of the Holt End boo boys.
O'Neill admits players of a weaker character would have quit Villa but Petrov slowly and surely won over his detractors in switching from an attacking to a holding midfield role.
Now it would be a major blow for Villa to take the field without him, which is why O'Neill was relieved when he announced his recovery from a virus.
O'Neill said: "It's really important for us to have Stiliyan fit. He's been really terrific and in the last two years he's been consistently brilliant. I'm delighted for him.
"When he came here, I envisaged him being the player I knew at Celtic. He was an attacking midfield player with Neil Lennon the one who did the holding to allow Stiliyan to get forward.
"Here, he started off like that and his debut against West Ham was as good a debut as I've seen.
"I thought we really had a player on our hands but he lost confidence and maybe just a change of environment was difficult to come to terms with immediately.
"He got a bit homesick for Glasgow - which is a bit strange considering he's a Bulgarian - and he took a long time to settle down.
"He had to overcome the wrath of the crowd and all of those things. That's why I've got the utmost regard for him because he could really have chucked it in and felt 'well this club's not for me.'
"But he was prepared to go and battle. I accept the fact that he's playing in a more holding central position, but he's done that so well over the last two years that he's deserved to be the captain."
O'Neill believes his crop of young English talent will not peak for a couple more years but knows winning a trophy will bolster their belief and confidence.
He said: "Winning a trophy is the ultimate. There is no getting away from that and it would be fantastic.
"Ideally that would be terrific - but the team this season has given great value for money. You have got wide players taking people on, there is good inter-change play, there is great movement, and these players are only improving.
"Stewart Downing has been a revelation, fantastic. James Milner has been great in a new role (central midfield) but, not just their enthusiasm, but their ability, has shone through.
"We are far from the finished article. United have got wonderful players - and so have we - and I think over the next two or three years that will be borne out.
"Players are improving, players we have just mentioned there, are improving all the time."
O'Neill accepts United will have the advantage of being accustomed to the big occasion when the teams meet tomorrow with Villa contesting their first final in 10 years since the 2010 FA Cup.
He said: "United have very good players and have been for some considerable time. They play big games, they know how to handle big games and, when you are playing for Manchester United, every game is a big game.
"That is the way they have been brought up. They go to cup finals like people eating breakfast. It is a real compliment to their team that they keep going all the time.
"They and Chelsea are stand-outs in that they want to go for everything."
O'Neill appears to have no major fresh injury worries.