"From my point of view, it is not a real plan, but simply a list of topics," Johann Wadephul told journalists in Brussels.
"I believe it is the task of the negotiating parties to define this."
Kyiv's EU backers have pushed back against the plan from Washington, and said Kyiv and Europe need to be on board.
"We want to ensure that Ukraine can discuss these points from a strong negotiating position," Berlin's top diplomat said.
"It will be Ukraine that decides what compromises it makes, just as Russia will have to make such decisions for its part."
He added that "we are not referees here, but we are advocates for Ukraine, because Ukraine is defending its freedom and the freedom of Europe."
Under the sweeping 28-point plan backed by US President Donald Trump Ukraine would give up a swathe of eastern territory to Russia and slash the size of its army.
Kyiv would also pledge never to join NATO, and would not get the Western peacekeepers they have called for, although European warplanes would be stationed in Poland to protect Ukraine.
Russia would meanwhile be readmitted to the G8 group of nations and be rewarded with sanctions relief under the plan.
The proposal involves major concessions by Kyiv, which has previously refused to cede any land, while appearing to meet many of Moscow's maximalist demands following its 2022 invasion.
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