Wirtz left Bayer Leverkusen for Liverpool in a £125m deal last summer, rejecting Bayern Munich in the process.
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The Germany international has since struggled to adjust to the demands of the Premier League.
But Voller told reporters this week: "I don't think the high transfer fee is the problem.
"That doesn't worry Florian. He certainly thought it would be a bit easier to find his place in the team. It takes time for everything to fall into place.
"Florian has to get over it."