The 18-year-old collected Lionel Messi’s pass on the edge of the area before cutting on his left foot and curling a wonderful effort into the top corner, doubling Barca's advantage after Jordi Alba's first-half strike.
What did Moriba say?
Moriba only made his senior debut in January against Cornella in the Copa del Rey, but impressed Koeman enough to stay in and around the first-team squad.
He made his LaLiga debut against Alaves at the start of the month, with Saturday’s clash against Osasuna just his fifth senior appearance.
“Leo gave me a good pass and I don’t know how I cut and shot with the left, and thank God it went in," he told Barca TV.
"I will never forget this and I will take it to my grave. There’s competition for places but it doesn’t matter if you are young or not. If you play well you’re in the team.
"The older players help us, give us advice, Leo tells me to get in the area as often as I can and get in positions to shoot.
“I want to dedicate this goal to my father and La Masia, where I have learned a lot from.”
Moriba also insisted that Barcelona had not given up on the title, despite leaders Atletico having two games in hand.
“La Liga isn’t over for us,” added the midfielder, who at 18 years and 46 days is the youngest player to score from outside the box in La Liga this season. "We will fight till the end. That is the DNA they taught us at La Masia.”
Moriba the future of Barca - Koeman
Koeman was full of praise for the teenager after the game, saying he believes he can be a Barca regular for years to come.
"Ilaix scored a goal in a difficult match, with the second goal we killed off the game. Every time Moriba enters the pitch he brings something extra,” said the Dutchman.
"Players like Pedri, Ilaix, Riqui [Puig], [Ronald] Araujo and [Oscar] Mingueza are the future of the club.
"Ilaix is young, but is physically very strong. He's gaining ground to be with the first team more often."