Despite a summer in which the club has lurched from crisis to crisis, Ronald Koeman has started the domestic season well, winning all of the competitive matches he has taken charge of.
"We have to be at our best; there is no other way against Barcelona," Lopetegui explained in his pre-match press conference.
"It's a different side with a new coach and a renewed energy, and they're showing good things.
"However, every match is an opportunity and we have to do what we want to do."
Lopetegui is also trusting sports chief Monchi to deal with any further transfer business.
"We have already said that the squad, in terms of ins and outs, depends totally on Monchi," he added.
"We are connected; he knows what I want and I fully trust him.
"I am focusing on the players we have available to help us in Sunday's match."
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