The midfielder, on loan from Barcelona, was substituted around the half-hour mark of Sunday's 1-1 draw with Real Betis at Riazor after a heavy fall onto his shoulder.
He asked to come off immediately, visibly concerned, and was replaced by Diego Villares. Deportivo La Coruna confirmed on Monday that he had since undergone successful surgery on the fracture.
Reports differ on the exact cause: some accounts describe a poor exit by Deportivo goalkeeper Leo Román that led to the fall, while others attribute it to a challenge from Betis midfielder Marc Roca.
The club's medical statement did not give a firm recovery timeline, saying only that his return would depend on his progress, but outlets including La Voz de Galicia estimate the layoff at around three months, which would keep him out until close to the turn of the year.
It is a significant blow for Deportivo and head coach Antonio Hidalgo, who was already downbeat about the injury as soon as the match finished. Casadó arrived in the summer as one of the club's most eye-catching signings and had just begun to establish himself in the starting line-up, with two starts in his last three appearances before the injury.
The 23-year-old made 49 appearances for Barcelona's first team after breaking through from the club's academy, and has two senior caps for Spain. Deportivo are looking at Noé Carrillo, who is himself recovering from an ankle injury suffered against Real Madrid, as a possible option to help fill the gap in midfield in the coming weeks.
Deportivo, promoted to La Liga last season, sit in the bottom half of the table and will now have to navigate the coming months without one of the players they were relying on most to help them stay up.