It was in the first half of Molenbeek's game against Beveren in April of last year that it happened. Molenbeek's best defender contested a duel with the opposition striker and emerged from it with a broken nose. The blood was streaming down his face.
"There was a lot of it," Jonathan Alves tells Sky Sports. Alves was Molenbeek's assistant coach and feared the worst, for player and team. "This was first against second and we were sure he was going to stop. It was impossible. But he just cleaned himself off."
Alves still remembers the moment that this Irishman turned to the Belgian crowd and urged them to roar their support. "Within minutes, he was delivering a top performance. If you want to know who Jake O'Brien is, just watch that game and see what he did."
It feels a little uncomfortable to praise such bravery when there would have been no shame in going off. But for those involved, this was a defining moment in Molenbeek's title win. "A big game, lots of pressure? Let's go. That is my best memory of Jake."
At the time, O'Brien's achievements overseas were no more than a curiosity to those Crystal Palace supporters following the fortunes of a player signed from Cork City in 2021. On his return, he would depart for Lyon for a fee of around £1m and that was that.
But such has been O'Brien's impact in his sole season in France that he is returning to the Premier League as a £17m signing for Everton. Now 23, he is ready for the next phase of his career and those who have worked with him expect him to shine under Sean Dyche.
His first steps in English football came on loan at Swindon Town in League Two. Ben Garner, then manager of the Robins, had been at Palace himself. "I could pick people's brains there, not only on him as a player but his character," Garner tells Sky Sports.
The reports were of a towering young defender who stood all of 6ft 6ins tall. "We were looking for a centre-half to give us that aerial presence in both boxes, particularly off set-plays, which we struggled with a little bit because we just were not a big side."
But there was more to O'Brien than his height.
"He had to be someone comfortable on the ball who could build attacks and Jake has got those attributes. He came in, brave as a lion with the ball, quick, good athleticism for his size, obviously good in the air with his frame, and a really humble, determined lad.
An injury towards the end of that season slowed his progress a little bit but the loan move to Belgium for the following season helped add more to his game. Within a handful of training sessions, it was obvious to Alves that he could make a difference.
"You could immediately see the impact that he had on the team. He was the best defender in the league, no doubt, and by far. We knew that he would be a top player at international level. That league was too small for him but it was also a good step."
As at Swindon, his aerial prowess was outstanding. As at Swindon, his former coach is keen to caveat the compliments about his physicality with a reminder that it comes with other qualities too. "He is tall so he is going to win all the duels in the air," says Alves.
"That is normal, of course. But what is particular about him is that he can also play with a lot of space behind him because he is fast too. That is very important in the top leagues. You need to be comfortable with space behind and he is very comfortable.
"For example, teams normally keep an extra player behind. If there is one striker, you are going to leave two defenders and so on. With Jake, that was not necessary. One against one was enough to give us the advantage. It was like we had two players there already."
Molenbeek often dominated their matches that season, one in which O'Brien featured 30 times. He relished it. "On the ball, he is very good. When he is building up under a high press, that is not a problem for him. He is not afraid to play. He can break the lines."
There were demands on all the team that season, not only to play at a high level but do so consistently. Promotion was the target from the outset. Despite being a young player in a new country, O'Brien took that challenge in his stride, welcoming the responsibility.
"The expectation was that the team would do well. That brings pressure, from the press, the fans and from the board. But one of his biggest strengths is his mentality. He has a very high self-confidence, which is important. He does not care what people think."
That attitude served him well at Lyon, where the circumstances upon his arrival were dire. The seven-time French champions were rock bottom when he made his debut in October and still there as Christmas approached before a spectacular turnaround.
O'Brien himself scored four goals, including a late winner at Toulouse. Improbably, he was Lyon's second-highest scorer behind Alexandre Lacazette. "I think he can do even better from set-pieces because he is tall, aggressive and reads the ball well," says Alves.
Dyche's Everton might just be the team to unlock that particular potential. "I think it is a good pairing because of their mindset. The Premier League has been his goal since the start and I think he is ready for it. He is confident but he is humble enough to listen too."
Garner shares that view.
"I have been so pleased to see what he has gone on and done since, because as I said, he is a really down-to-earth, hard-working and determined lad. I think he has all the tools to go on and have a really successful career, as he is showing at the moment.
"I think his progression has probably been quicker than anyone would have imagined but it just goes to show that if you do get those decisions right in terms of your loans and the moves that you make, that environment can really help you to thrive quite quickly."
There has just been the odd bloody nose to deal with along the way.