Manuel Pellegrini said it was a bittersweet feeling after Real Betis beat Sevilla 2-1 in a Copa del Rey derby that spanned two days and was marred by violence.
The Betis head coach saw his team resume at 1-1 on Sunday at Estadio Benito Villamarin, and a strike from Sergio Canales carried them through to the quarter-finals.
The game had been suspended on Saturday due to crowd trouble, with Sevilla midfielder Joan Jordan hit by an object from the stands during celebrations of Nabil Fekir's 39th-minute goal that equalised Papu Gomez's opener.
Jordan was taken to hospital for checks after being struck by a projectile from the crowd that appeared to hit him on the head.
After the game was completed behind closed doors on Sunday, television footage emerged of Betis substitute Andres Guardado seemingly mocking Jordan's misfortune while celebrating, bouncing a water bottle off his forehead and slumping to the ground, feigning injury.
The incident drew an immediate rebuke in the Spanish media, where it was widely interpreted as a jibe at Jordan.
Pellegrini said of the result, according to AS: "The first thing is that I'm happy to be in the next round. But also bitter because I think a lot of damage has been done by a brainless person [when Jordan was hit on Saturday].
"I don't know if a plastic tube does that much damage, but there has to be an investigation. Football suffered great damage."
Sevilla boss Julen Lopetegui did not want the game to resume on Sunday, and said: "There were many possibilities to play on other days. Justice in this case does not exist at all."
Canales put Betis ahead in the 73rd minute when he was left in space on the left after a pass from William Carvalho and surged forward before beating goalkeeper Alfonso Pastor.
Jules Kounde hit the bar with a late header as Sevilla battled for a leveller.