Mariano has been a sensation for Lyon this season, having only left Real Madrid in August for France.
Lopetegui admits he's now having the young striker watched as he considers the makeup of his World Cup squad.
"Mariano? He is Spanish and selectable. We went to see him and we are watching his evolution," said the La Roja boss.
Mariano has been capped at senior level by the Dominican Republic, but as it was a friendly he remains clear to play for Spain.