FOXBORO — When his opportunity arrived, Justin Rennicks got lost in the moment.

The United States U-20 team trailed France 2-1 in its World Cup Round of 16 match earlier this month when Rennicks, a rookie forward with the Revolution, came on in the 63rd minute. The Americans tied the game soon after, and then Rennicks’ time came.

In the 83rd minute, U.S. defender Sergino Dest took a shot from outside the 18-yard box, and Rennicks followed the ball. French goalkeeper Alban Lafont’s save took a rebound, and Rennicks was there at the far post to bury it.

Delirium ensued.

The goal — the decisive tally in the Americans’ stunning 3-2 victory that pushed them into the World Cup quarterfinals — and the gravity of the moment overwhelmed Rennicks so much that he didn’t know what to do, or how to react, as his teammates mobbed him.

“It’s funny,” Rennicks said. “I had a whole celebration set up, and the goal was just so meaningful to me and it just meant so much, especially to take the lead on France to move on in the tournament, I didn’t even get to do my celly.

“I was just looking around, like what’s going on? Did that really just happen? It was one of the greatest feelings I’ve had in a while. It’s hard to explain, but it was amazing.”

The goal capped what’s so far been a surreal year for Rennicks, who is a native of South Hamilton. After spending more than four years in the Revolution Academy, where he was named the Academy Player of the Year in 2014 and 2015, Rennicks signed a homegrown contract with the Revs in January.

Rennicks made his MLS debut in early March as a substitute, but a hamstring injury he suffered during national team camp threatened his availability for May and June’s U-20 World Cup. Fortunately, though, it was just a hamstring strain grade 1, which Rennicks said was the best outcome for that type of injury, and he fully recovered by the World Cup.

“I got lucky, not gonna lie,” Rennicks said.

Though Rennicks said he didn’t get the minutes he wanted at the World Cup, the 20-year-old was still thrilled to get the call-up and make an impact. In the fall, he scored four goals across six appearances as the U.S. won the CONCACAF U-20 championship, which helped earn him a spot at the World Cup, where he seized his opportunity against France.

Now, he’s hoping to build on that performance and carry that strong momentum into training with the Revs.

Rennicks made his debut on March 9 when he came on as a second-half substitute and played 45 minutes in a 2-0 loss to Columbus, but he hasn’t seen the field since. Though he’s shown well for years at the youth level, it will likely take some time before Rennicks cracks the 18-man squad for the Revs on a consistent basis as he continues to adjust to a different level.

The difference in playing style in MLS is something even he knows will take time to get used to.

“They’re all men,” Rennicks said. “I can call myself a man, but they’re like much older than me and they’re bigger than me, so you’re playing a physical game now. It’s not how academy used to be. You’re going to take some cleats every now and then, that’s just how they play. It’s a much more physical game, a game that I’ve been waiting to play for a while, and I’m willing to continue to play.”

Patience will be key for Rennicks as he continues to work for his next opportunity.

New Revs coach Bruce Arena is still learning all of his players, and Rennicks is no exception as he continues his steady development. Arena has certainly had conversations about Rennicks with U-20 coach Tab Ramos as well as Curt Onalfo, the Revs’ new technical director who spent time with the U-20s. It remains to be seen how he fits into the Revolution’s plans.

“We have a pretty good idea about him, but we need to see him now in this group,” Arena said. “Going from the U-20s to senior football or soccer, however you want to describe it, is different. His physical qualities that are very dominant at the youth level are a little bit different when he plays with adults, as I like to say, so in time, we’ll get to know him a little better and he’ll have to start adjusting his game to this level of play. But he has all the tools to become a very good player.”

If he does, there should be plenty more chances to celebrate in his future.

“Not getting on the field is the worst thing, but I’m out here every day,” Rennicks said. “I’m just going to keep trying to get in the starting 11, no matter what it takes.”