The president-elect has consistently said his priority is to end the war and stem what he characterises as a drain on US resources, in the form of military aid to Ukraine.
However, he has yet to divulge how he intends to do so – and will likely be hearing competing visions for Ukraine’s future from his various advisers.
Mr Lanza, Trump’s political adviser since his 2016 campaign, did not mention areas of eastern Ukraine, but he said regaining Crimea from Russia was unrealistic and “not the goal of the United States”.
“When Zelensky says we will only stop this fighting, there will only be peace once Crimea is returned, we’ve got news for President Zelensky: Crimea is gone,” he told the BBC World Service’s Weekend programme.
“And if that is your priority of getting Crimea back and having American soldiers fight to get Crimea back, you’re on your own.”
The US has never deployed American soldiers to fight in Ukraine, nor has Kyiv requested American troops fight on its behalf. Ukraine has only requested American military aid to arm its own soldiers.
Mr Lanza said he had tremendous respect for the Ukrainian people, whose “hearts are made of lions”. But he said the US priority was “peace and to stop the killing”.
“What we’re going to say to Ukraine is, you know what you see? What do you see as a realistic vision for peace. It’s not a vision for winning, but it’s a vision for peace. And let’s start having the honest conversation,” he said.
Trump is expected to handle peace talks with a close circle of aides once in office.
An unnamed National Security Council aide who previously served under Trump told the Wall Street Journal on Wednesday: “Anyone – no matter how senior in Trump’s circle – who claims to have a different view or more detailed window into his plans on Ukraine simply doesn’t know what he or she is talking about.”
They said that the former president “makes his own calls on national security issues” and had done so “many times in the moment”.
Trump spoke to Zelensky after his election win, with billionaire Elon Musk also taking part in the call.
A source in Ukraine’s presidential office told the BBC that the “good lengthy conversation” between Zelensky and Trump lasted “about half an hour”.
“It was not really a conversation to talk about very substantial things, but overall it was very warm and pleasant.”
Trump’s Democratic opponents have accused him of cosying up to Russian President Vladimir Putin and say his approach to the war amounts to surrender for Ukraine that will endanger all of Europe.
Last month, Zelensky presented a “victory plan” to the Ukrainian parliament that included a refusal to cede Ukraine’s territories and sovereignty.
During his election campaign, Trump repeatedly said he could end the war between Russia and Ukraine “in a day”, but never gave further details.
A paper written by two of his former national security chiefs in May said the US should continue supplying weapons, but make the support conditional on Kyiv entering peace talks with Russia.
Ukraine should not give up its hopes of getting all of its territory back from Russian occupation, the paper said, but it should negotiate based on current front lines.