"In terms of where we are as a squad I don't think we could be in a better shape in terms of the injury situation and the preparation of the team, and where they are going into this game."
The team's top scorer in the qualifying campaign, with seven goals, Lafferty had sat out Wednesday's training a day after picking up the injury.
The 28-year-old was back in training on Friday, however, and told the BBC he was feeling good after initially fearing the worst.
"When it happened, I had the sharpest pain in my groin," he said. "But as that day went on, it started feeling better and better. I came through it and then got the scan results which were all clear."
Sunday's match will be Northern Ireland's biggest in 30 years as their first appearance at a major tournament since the 1986 World Cup in Mexico.
O'Neill said his men were ready for it.
"The players are very well prepared. I don't think there's a need for a final message. They understand the significance of the game.
"I think the preparation we've done has been flawless, basically. I don't think I've ever felt that a team has been as well prepared in the four years that I've been in charge, in terms of what we've managed to do with the squad."
(Reporting by Alan Baldwin, editing by Ian Chadband)
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