The Bulgarian playmaker has performed in fits and starts for Villa since joining the club in a big-money deal from Celtic last summer.
"Stiliyan Petrov is going through a tough time at this minute, but he'll come back," O'Neill told the Birmingham Mail. "The crowd have got more than anxious at a time when he maybe needs them behind him, just to settle him down.
"But players have to come through these things - I had it myself in my career and every player gets it at some stage or another.
"It's been suggested that I've put Stiliyan in front of some of the other lads simply because I'd bought him.
"The bottom line is I really couldn't care less who's in the team - if the players are doing it for me, then I couldn't care less whether he cost £6.5million or £200,000.
"No-one has a divine right to be in the team. I'd hope that at some stage the same would apply to Gareth Barry, but he's so good for us at the moment that he's in.
"It's got nothing to do with justification. I left Stiliyan Petrov out of the team at Celtic and for some European games. But he always came roaring back."
