World Cup winners in 2014, Joachim Low's side offered a meek title defence in Russia as they bowed out at the group stage, and they were subsequently relegated from the Nations League top tier having finished bottom of a group featuring Netherlands and France.
Low caused controversy earlier this month by dropping Thomas Muller, Jerome Boateng and Mats Hummels from his squad, while Die Mannschaft were booed off at half-time in Wednesday's 1-1 friendly draw with Serbia in Wolfsburg.
Koeman, who saw his side stretch their unbeaten run to five matches with a 4-0 win over Belarus in their opening Group C game on Thursday, is adamant that Low's men should not be underestimated and he sees some similarities with his own situation when he took charge of the national team last year.
"If you don't lose for a long time, there will be a moment you do lose, even though we would like to stay in our winning ways," he told a media conference.
"If you compare the two squads, it's more clear on our side. Our players look more confident, sometimes a bit too arrogant against Belarus. But we do realise that we have to be at our best because Germany has really good players that can make our lives miserable.
"Of course, there's some parallels. They're rejuvenating, they've tried different formations like I did. I too started with five defenders, but we switched to a different system, which we will keep for now.
"We've tried to develop young players, that's what's Germany is doing now as well. Maybe it's easier to just start with new players than having to say goodbye to older players - that's what's happened in Germany. Muller, Boateng and Hummels still play at the highest level. But that's their problem."
Memphis Depay was instrumental in the emphatic win over Belarus, scoring twice and creating the other two.
"I don't do anything special with Memphis, I only give him confidence," he added.
Meanwhile, Virgil van Dijk believes the clash with Germany will be "a special game" and targeted a repeat of the 3-0 Nations League triumph his side enjoyed in October.
The Liverpool defender opened the scoring at the Johan Cruijff ArenA when Koeman's men brushed Germany aside, Depay and Georginio Wijnaldum adding late goals in a comfortable victory.
Looking ahead to Sunday's game, Van Dijk told reporters: "It's a special game with lots of history. It's always got that extra thing, that's what everybody should feel.
"I do feel that myself. We're now in a period where points do matter a lot. Tomorrow will be a special game.
"I think you always need to prepare well when playing for your national team or your club. But this time you need to do that. I'm not worried about that at all, I'm sure all my players will be well-prepared."