However, the teenager swiftly made amends as he ran the ball back from his dangerzone to clear the halfway and then draw a penalty in the tackle as the French desperately tried to regather possession, before taking the tap and ducking a tackle to kick the ball dead for fulltime.
No doubt impressed with Koroi's progress, NZ coach Scott Waldrom introduced him into the starting lineup and the crucial playmaker role for the evening game with the United States.
Koroi repaid the faith with New Zealand's opening try - from a stop-start position he turned on the gas to step around one tackler and then step out of the diving tackle from a second to score under the posts, with his conversion levelling the scores at 7-7.
Koroi then immediately saved a try at the other end with an excellent cover tackle around the legs, although the Americans scored soon after while New Zealand were a man down with Tim Mikkelson in the sinbin.
Koroi made another good cross field run early in the second half, and after he was replaced the veteran New Zealand players took over to run out 24-12 winners.
The teenager told Sky TV he had been "pretty emotional" to get the news he would represent his country.
"I didn't know why they were really calling me, but got the call up so just took the opportunity by hand.
"Now I'm here, so I'm very thankful for that."
Koroi said he was so focused on getting the ball down for his first international try that he "went blank" after doing it.
But there was no question the young man had enjoyed his Saturday in the capital.
"Pretty excited to be out here, having my first start, [and] first try as well so it's pretty good."
Koroi also acknowledged the help he had received from the squad veterans like Mikkelson and DJ Forbes.
"Those boys are very good, they really help me around, help all the new boys.
"Even though they always help each other out as well, so it's very good to have them on the field and off the field as well."
New Zealand's run to the title was derailed by handling errors in the quarterfinal with Fiji, losing 26-10.