Celta won through a doubt from Williot Swedberg, with Real having Fran Garcia, Alvaro Carreras and Endrick all sent off.
Xabi said afterwards: “We're all angry, obviously. It wasn't the match or the result we wanted. From the start, Militão's injury hurt us. It took us a while to recover mentally. Then we made some adjustments, but it wasn't the match we wanted.
"We have to try to move on as quickly as possible. It's only three points, there's still a long way to go in the league. On Wednesday, we have a Champions League match against (Manchester) City to react and remove this bad taste from our mouths.
“We'd planned the match after the Bilbao game to play at a good pace and put pressure on a good side. We were lacking in some areas, and the injury disrupted our plans. It's disappointing; we're suffering quite a bit with injuries. It's another very important one to add to the list.
"We struggled to react; almost the best reaction was when we were down to 10 men. The team pressed hard, we ran and we fought. We understand that people left angry. On Wednesday, we have to show another version of ourselves.”
Xabi also insisted: “Everyone is united, we know that this is football and that the situation can be turned around. There is still a long way to go in the season and a bad game at home can happen. It's not what we want, but we have to look ahead. We know what this is all about: self-criticism and high standards. We know that defeats hurt a lot.”