Isak and Wirtz arrived at Anfield in a pair of deals worth a combined quarter of a billion euros, but both struggled in Sunday's 2-2 draw at Newcastle, a match new manager Andoni Iraola's side only salvaged through a stoppage-time penalty.
Gary Neville was unsparing in his assessment afterwards.
"The big thing for Liverpool is they've got this quarter-of-a-billion-pound front two, Isak and Wirtz, and bloody hell, we've been kind to those two," he said. "They're going to have to start playing sooner or later or else, to be fair, they're going to get quite a lot of criticism."
Liverpool captain Virgil van Dijk offered a more sympathetic view of Isak's start, insisting the Sweden international had impressed him throughout pre-season despite the criticism.
"In my opinion he had a fantastic World Cup. He had a very good pre-season. He is in form and looks very sharp in training, and I have full confidence that he will become very important for us," the 35-year-old told the Liverpool Echo.
"As I said, he wants to succeed. It's up to all of us to make him successful, but it's also up to him to succeed. We have to be together."
Van Dijk also dismissed the idea that the jeers Isak received on his return to St James' Park, following his acrimonious exit from Newcastle last year, had affected his performance.
"I don't think it affected him. I wouldn't be affected by that if I were him. I don't think he was affected by it. He tried to play his game. There were good moments and there were moments he could have done better, just like all of us. I don't think it affected him," he said.
Elsewhere at Anfield, midfielder Harvey Elliott is attracting interest from Napoli and Lazio, though Liverpool have yet to receive a formal enquiry for the 23-year-old. Elliott endured a difficult loan spell at Aston Villa last season, complicated by a clause that would have made the move permanent for £35m had he reached ten appearances.
Unai Emery opted not to select him after a handful of games, a decision Elliott admitted had affected him at the time. He made just nine appearances and scored once for Villa, though he did finish the campaign with a Europa League winners' medal as an unused substitute.
With four days left before the transfer window closes, interest in Elliott is expected to intensify, even without a concrete approach yet on the table.