That plan backfired as Newcastle shipped five goals against Crystal Palace at the weekend, giving them the division's worst goal difference.
"Nobody is happy here. We've lost the last two games and not played very well. Confidence can go as quickly as it comes," McClaren told reporters at his pre-match news conference on Thursday.
"The next game is always an opportunity to build a performance, get a result and the confidence can come back.
"The fans can be so influential and we need everyone together (on Sunday) no matter what.
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"The mood is focussed and determined, and we know that we can turn it around with one result because we have good players."
Newcastle welcome a Liverpool side who have scored 14 goals in their past four games away from Anfield in all competitions and McClaren says their newly-installed German manager has a clear plan that is beginning to bear fruit.
"Juergen Klopp is a quality coach who I know from Germany. I can see what (Borussia) Dortmund had in the Liverpool team. It's coming."
Newcastle have the added headache of an injury to centre back Mike Williamson, who is set for a scan on a hamstring problem he picked up in training.
McClaren recalled the 32-year-old from a loan spell at Championship (second tier) side Wolverhampton Wanderers this week following an injury to regular starter Jamaal Lascelles.
(Reporting by Simon Jennings in Bengaluru; editing by Toby Davis)
