The defender, who joined the Spanish topflight side on a season-long loan from Chelsea, replaced injured Unai Bustinza in victory, which came about thanks to an early strike from Guido Carrillo.
Despite the victory, Mauricio Pellegrino's men sit in the relegation zone with seven points from eight matches.
Leganes, desperate to make amends for their 1-0 defeat to Real Betis in their last outing, were on the attack from the off and opened the scoring with just 14 minutes on the clock. Carrillo did well to head home a cross from Michael Santos.
His fine showing at Estadio Municipal de Butarque will be a morale-booster for him as he heads to Super Eagles’ camp for their double-header Africa Cup of Nations qualification against Libya.
Excited by his long-awaited bow, Omeruo took to Instagram to share his excitement.