Wirtz joined Liverpool from Bayer Leverkusen in the summer for 125 million euros ($134 million) plus bonuses, but has not stood out despite the Reds winning their first three matches.
Speaking to reporters in Bratislava before Germany's World Cup qualifier against Slovakia on Thursday, Nagelsmann called the 22-year-old midfielder "a key player for us".
The coach said Wirtz had "not been incredibly dominant" and added he needed time to adjust at Liverpool.
"You have to look at the whole gang playing around him. They've spent a few euros on the top squad they've got there, and you don't always stand out as an individual.
"Rather, it's the overall construct which should work - and it does work.
"It's normal that you need a bit more time in a new culture, a new league, in a new stadium at a new club.
"He'll play some great games there. He'll score, he'll assist. He can start that with us tomorrow," he added.
Along with Wirtz, the Premier League attracted several Bundesliga stars in the summer, with four of the top five most expensive transfers coming from Germany.
Germany captain Joshua Kimmich, who is one of Bayern Munich's top earners, said players were "torn".
"To be honest, we can't really grasp it either, and we can't influence it. But it's the market.
"There are financial resources there (in England), both through ownership and TV money.
"Apparently, they can pull the money out of their sleeves very, very easily."