It did not look good. Gingerly, Lafferty rose to his feet and with the help of Irish FA medical staff hobbled away from the pitch.
As he entered the changing room, all those watching the 28-year-old from Fermanagh wondered if his Euro 2016 race had been run without him even reaching as far as the starting blocks.
And to think before this everything had been going so well. Perfectly, in fact.
There had been three worthwhile training camps in Manchester, Dublin and Austria and two friendlies in which Northern Ireland enjoyed the ideal Windsor Park send-off, beating Belarus 3-0 with Lafferty, Conor Washington and Will Grigg on target, followed by a battling 0-0 draw in Slovakia to extend the record-breaking unbeaten run to 12 matches.
On arrival in France, the players loved the team hotel and the facilities.
All was going according to O'Neill's masterplan.
Then yesterday at Saint-Gerges-de-Reneins, just outside Lyon, with hundreds of locals enthusiastically waving specially made Northern Ireland flags during the training session, the juggernaut was stopped in its tracks when Lafferty lunged for a ball in the type of challenge he has made most of his life.
Everyone feared the worst. Within seconds, word was spreading around social media and a 'Pray for Kyle' hashtag was doing the rounds on Twitter.
Around 20 minutes later, the news from the dressing room was that Lafferty felt fine.
Relief all round, but surely too early to confirm that the man who scored seven goals during the successful qualifying campaign - which saw Northern Ireland top their group after coming in as fifth seeds - was going to be fit and well to take on Poland in Sunday's opening Euro 2016 clash in Nice.
This was drama O'Neill could have done without.
But addressing the press after the training session was over, the Northern Ireland manager revealed that the Norwich City striker was fine and dandy after all.
"He's fine, it's precautionary. We have no major concerns there," said O'Neill.
"It was a little twist (of the groin) but on examination we don't expect any issues at all. We're being careful.
"He just twisted it a little bit as he slid.
"The pitch is hard and you're always liable for these little situations to happen in training.
"He seems alright. He's probably a little bit scared to get in the ice bath but he's there every day. He's fine. All good."
And with that, O'Neill made his exit as Lafferty climbed into the ice bath in the distance.
Prayers had been answered, though it will be interesting to see if there is any reaction to the injury and if the former Rangers front man will take part in training today.
The incident with Lafferty also shows just how thin the line can be between a footballer living out his dream and playing in his country's first major tournament for 30 years and experiencing a nightmare and missing out altogether through sheer ill fortune.
Lafferty is one of Northern Ireland's key performers at Euro 2016.
The manager, players and fans know that the team needs him out there against Poland.
The incident was still the talk of the media centre when the press conferences began later on.
When asked about Lafferty, fellow forward Washington played the moment down, insisting that for players it was run of the mill stuff.
"There are obviously going to be scares when people go down in training but we are told he's going to be fine and it was just a precaution," said Washington.
"It was another good day's training for us. It didn't affect the session.
"As a professional footballer you see that day in and day out.
"Him coming out was perfectly understandable.
"You don't want to take risks ahead of a massive tournament like this."
Asked if it was more serious than people thought, would he be ready if he had to step up and replace Lafferty, Washington's reply was: "I'm definitely 100 per cent ready to go."
Northern Ireland supporters will hope Lafferty is the same.
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