The Dutch were last in action last Tuesday in Berlin where they went down 3-2 to Austria in their final Group D game, relegated to third place in the standings but still advancing to the knockout stage.
"It has been a very long week. Seven days is a long period between two matches at a tournament like this," Koeman told a press conference.
"We’ve certainly had enough time to review and analyse the match against Austria, and what went wrong. We didn't play compact enough, which sometimes makes it seem like you're not aggressive enough."
The Dutch had long discussions and extensive critique after the performance.
"Players and staff were open with each other. After that, you continue and look for a reaction in training. Everyone knows that things can and should be done differently and now we look forward to seeing the confirmation of that tomorrow," he explained.
Koeman’s decision on who plays in the midfield is eagerly anticipated, with changes expected but he gave nothing away to reporters.
But he said he would insist on a more compact lineup, better aggression and communication.
"And the creative players must be able to make a difference,” he said.
Injuries to key midfielders like Frenkie de Jong, Marten de Roon, and Teun Koopmeiners have seen Koeman struggle to find the right combination in the middle.
"It's a long story and I don't like to talk too much about who is missing. This is the squad that we have and we need to improve, but it's not only the midfield but sometimes the communication between the centre-backs and the midfielders needs to be better."
Koeman said the Dutch had plenty of motivation for the clash at the Munich Football Arena, where they will be favourites despite a mediocre return at Euro 2024 so far.
"We are a proud nation. We like to win, we like to play nice football, but that doesn’t always happen. We take the criticism… no problem… and we'd like to turn it around tomorrow and make people happy about the final result.
"We take Romania really seriously; we know what we can expect and we need to improve compared to the last match.”