Spanish media reported that armed men entered the Hungarian goalkeeper's house at night while Gulacsi was there with his wife and their two children.
It is stated that four masked intruders broke into their home with pistols in what will have been a horrifying event for him and his family.
The club announced ahead of kick-off that Gulacsi was unavailable "due to personal reasons", with Luiz Junior stepping in to take his place.
Spanish authorities are investigating the robbery. Reports said the suspects had not been apprehended at the time of publication and are still at large.
Villarreal won 3–1 despite his absence and the Spanish side now sit 10th in the Champions League table, with a whole host of ties still set to be played on Wednesday night.
His former sides Liverpool and RB Leipzig face off against LASK and AC Milan respectively. Gulacsi will still be shaken from the events and will hope that his possessions can be recovered as soon as possible.