The sight of the influential midfielder leaving the field with blood pouring from his face took the gloss off Celtic’s return to the top of the Scottish Premier League.
But O’Neill believes the Bulgarian could face Celta at Parkhead on Thursday.
“I think he had six stitches outside his mouth and nine inside his mouth,” said O’Neill.
“That’s what was causing him the difficulties and Roddy Macdonald, the doctor, said he would be able to sort it.
“But it would take as long as 20 to 25 minutes and I decided that was too long so I brought on Jackie McNamara.
“He’s feeling not too bad, but a bit dazed-looking. I’m sure he will throw himself into contention for Thursday.”
Petrov was injured after colliding with Marvin Andrews as referee Stuart Dougal awarded Celtic an early penalty, which Henrik Larsson fired home emphatically.
Larsson eased the champions’ nerves late on with a great finish after Andrews had given the visitors a scare with a header against the crossbar.
O’Neill expressed his relief to go a point clear of Rangers again, but admitted the performance would not strike fear into the watching Celta scout. “Obviously I was very pleased to see the second goal go in, but it was a long second half for us,” he said
“They gave us a couple of anxious moments before Henrik scored a great goal to take the three points.
“They had some reasonable chances and we certainly might have been more comfortable after the first half, but I was pleased with the result more than the performance.
“I can’t imagine that performance would cause too much concern for the watching Celta contingent, but I am happy to be back on top.”
Livingston manager Jim Leishman has challenged his side to produce their second-half display every week to clinch a top-six place.
He said: “In the second-half there was no difference between us and them.
“I believe if we can do that if we can do that against other teams in the bread-and-butter games then we will be in the top six. I felt we were unfortunate not to have got at least a point.”
LIVINGSTON: Broto, Brinquin, Rubio, Andrews, Bollan, Bahoken, Makel, Quino (Dadi 66), Toure-Maman, Wilson (Zarate 74), Bingham.
CELTIC: Douglas, Valgaeren, Balde, Laursen, Sylla, Thompson, Lambert, Lennon, Petrov (McNamara 10), Larsson, Sutton.
Referee: S Dougal (Scotland).