The 23-year-old had initial been included in the Basque side's squad for the game, but was suddenly withdrawn on Wednesday amid speculation he was suffering from illness.
However, the seriousness of the situation soon became clear when the club released a statement reading: "Luca Sangalli suffered a mild stroke this morning.
"He is currently in the stroke unit of the Donostia University Hospital, where he will be studied."
Sangalli came through the youth system at Sociedad and has featured in six LaLiga games so far this season having made his debut against Eibar back in August.
The attacking midfielder has recorded one goal and two assists in his Real Sociedad career to date.
Clubs from across Spain quickly took to social media to wish Sangalli a speedy recovery and return to football, including Basque rivals Athletic Bilbao, who wrote: "Get well soon, Luca.
"We hope to see you soon on the pitch, even it means you hurting us again. Stay strong."