The moment came in the 76th minute of a Concacaf Champions Cup rout of Trinidad's Defence Force, Sullivan cutting inside from the right and drilling a left-footed strike past the goalkeeper to make it 6-0.
He added a second in the 88th, plus two assists either side, becoming the youngest American ever to score in the competition at 16 years, four months and 29 days old.
"I'm thankful for the opportunity, and happy to have scored my first goal, two of them, actually," Sullivan said afterwards. "More importantly, though, is the clean sheet, the win, and that we advanced."
Bradley Carnell, in charge that night, called it simply "another moment in the right direction."
It was the latest entry in a record book Sullivan has been rewriting since he was 14. He debuted for Philadelphia in July 2024 at 14 years and 293 days old, breaking Freddy Adu's 20-year-old MLS record by 13 days.
Jim Curtin, the Union's head coach at the time, had no doubts about what he was watching.
"He's special and you could see that tonight," Curtin said. "He's going to get better and better every time he gets minutes. He's going to get better and better as he moves up the levels and eventually moves to Europe. But while we have him, let's enjoy it."
On another occasion, Curtin went further still: "A special talent, top talent not only in this country but in the world."
The signs were there long before his MLS debut. In the 2023 Generation adidas Cup, a 13-year-old Sullivan helped the Union's academy side beat Valencia, Arsenal and Real Madrid en route to the final, scoring a minute into the win over Arsenal and earning a place in the tournament's Best XI.
That same year he was named Golden Ball winner as the United States won the Concacaf Under-15 Championship for the first time in the tournament's history. By 2025 he was starting games at the Under-17 World Cup in Qatar, finishing with a goal and two assists from three starts.
None of it happened by accident. Sullivan is the youngest of four brothers steeped in Philadelphia soccer across three generations — his grandfather Larry once ran the program at Villanova where Curtin himself played, his uncle Chris Albright served as the Union's technical director, and his eldest brother Quinn has anchored the first team since 2021.
"I remember coming into training with my uncle and I just looked up to all those guys," Cavan said. "I was inspired immediately. Then when Quinn finally signed, it just felt like a dream."
He signed his own record-breaking Homegrown deal — the richest in MLS history — at 14, telling reporters: "This is my home. It's an honor to sign here."
Then-sporting director Ernst Tanner, who brokered that deal, reached for one of the biggest comparisons in his scouting career: "Cavan, in my eyes, is among the top three talents that I have seen in my 30 years," comparing the signing to landing Dominik Szoboszlai as a 14-year-old at Salzburg. "He has all the possibilities to become a world star."
The style on the pitch backs up the boldest comparison being made about him: left-footed, deployed on the right, at his most dangerous cutting inside onto that stronger foot before either shooting or slipping a pass through the space he's just vacated.
It's a mirror of prime Arjen Robben's classic geometry — the same inverted angle of attack, the same low centre of gravity that lets him change direction without losing speed — though Sullivan, still filling out at 5ft 7in, is playing that role as a teenager against grown men, not as the finished article Robben was at Bayern Munich and Real Madrid.
That trajectory now points to Manchester City, who inserted a clause into Sullivan's contract to sign him when he turns 18 at the end of 2027.
Adu, the man whose record he broke, has become an unlikely mentor from afar, publicly mapping out the loan pathway he thinks City should follow.
"I really hope that it's to a team that he will be a regular starter for," Adu said. "There's nothing worse than a young player going on loan to a lesser team and not playing."
Adu is nonetheless bullish long-term: "Eventually he's going to crack the starting line-up for a Premier League team. I know it's going to be tough — it's probably the hardest league in the world to crack — but I've seen how good he is."
Adu's caution comes from experience — he was 14 himself when he was cast as the future of American soccer.
"Sometimes players are propped up too early, and they forget what got them there," he said. "I hope he has the right people around him to help him through all that."
Not every voice in American soccer agrees on the timeline. Apple TV analyst Taylor Twellman argued this year that Sullivan wasn't being used properly at club level: "There's a legitimate argument that Cavan Sullivan does not fit in the Bradley Carnell system, or his identity."
Fox Sports pundit Alexi Lalas takes the opposite view on the pressure itself: "You're not going to break him. He's good enough that that pressure from an early age is a good thing."
The dream that started it all, Cavan admits, was watching Belgium's Hazard brothers on television with Quinn.
"We looked at each other like, 'that could be us one day,'" he said. It very nearly has been — the pair have now shared a Union first-team pitch, with Quinn calling their chemistry "unmatched."
Union captain Alejandro Bedoya has taken on his own share of the mentoring since, texting the 16-year-old this summer after watching the World Cup from the broadcast booth: "I hope when you're watching this game, you see yourself here, because you're the future, right?"
For now, Sullivan remains a Philadelphia player with two years left to make his mark before Manchester waits. Judging by his last outing in blue and gold, that clock is not one he plans to waste.