The youngster scored the decisive spot kick to give the Catalans a 3-2 shootout win to book their place in the Supercopa de Espana final, with the game having finished 1-1 after 120 minutes.
"I had four names on my sheet," Koeman said after the game. "I asked who wanted to take the fifth and Riqui said 'I will'.
"He did very well."
Frenkie de Jong opened the scoring on the night with another goal, fulfilling Koeman's wishes to see him more involved in the final third.
"I've said a lot the the midfielders have to arrive in the box," Koeman added. "He's on the right track."