The Dutch thumped Scotland 4-0 in Amsterdam on Friday but Koeman was far from pleased with what he believed was a sloppy performance despite the emphatic scoreline.
He says that unless they tighten things up, the Dutch will put themselves under unnecessary pressure against a German side buoyed by an impressive 2-0 win over France in Lyon in their previous outing.
"I want us to play better in possession and to give away fewer opportunities," Koeman told reporters. "We suffered an unnecessary amount of ball loss, by multiple players. It was disappointing.
"We would be very stupid to (leave) players like (Germany midfielder) Toni Kroos as France did, then every ball will come to the right place."
The Netherlands have a quintet of Bundesliga-based players in their squad - Xavi Simons, Donyell Malen, Wout Weghorst, Jeremie Frimpong and Matthijs de Ligt.
Koeman must decide whether to stick with what he called three "real midfielders" against Scotland or revert to the Netherlands' more regular formation of two holding players and a more attacking number 10, likely to be RB Leipzig’s Simons.
"I never pin myself to one system, not even at Barcelona or Ajax," he said. "Sometimes that brought me success and sometimes less. You must have players who can play different systems."
There could be another run out for midfielder Georginio Wijnaldum, who returned to the side for the first time in nine months in the win over the Scots.
Koeman was pleased with his performance but would not reveal whether the 33-year-old had done enough to win a place in his Euro 2024 squad.
"We are not ready yet (to say) but I think Wijnaldum played more than a sufficient game."
