The Switzerland international admitted the Black Cats were punished for failing to execute the basics despite preparing thoroughly for the encounter.
"It's hard to take. We prepared well and came here with belief, but if we don't do the things we are speaking about, there is no chance to win any game," he told club website.
"There are so many mistakes from our side. If we don't do the basics, forget about everything else."
Despite acknowledging the growing frustration among supporters, Xhaka urged his teammates to remain united and focused on improving results.
"It's tough on the supporters – I know they are frustrated and it's the same for us,” he added.
"We have to stick together and believe with each other. We need everyone and I believe we will bounce back."