It's unlikely he would want to rewrite the whole deal, said Schott, who's also a member of two government advisory committees on trade.
In a wide-ranging press conference in NY on Wednesday, Trump also said, "We are very unhappy with the negotiations and the negotiating style of Canada".
U.S. President Donald Trump says he rejected a request for a one-on-one meeting with Prime Minister Justin Trudeau to discuss NAFTA this week because Canada's tariffs are too high and the country's negotiators have refused to budge.
The U.S. and Canada are under pressure to reach a deal by the end of the month, when Lighthizer must make public the full text of the agreement with Mexico.
Earlier in September, Trump tweeted that there is "no political necessity to keep Canada in the new NAFTA deal".
It is possible that Congress, which strongly prefers an agreement involving Canada, would allow Canada to be added to the U.S. -Mexico text at a later date. "Canada will come along", said Trump who added that if they don't make a deal, the USA will "make a much better deal" by taxing Canadian auto exports to the U.S.
Trudeau also downplayed a brief handshake with Trump at a luncheon Tuesday, where the US president remained seated despite standing to greet other leaders.
Despite efforts to get Canada on board with a new NAFTA deal by Sunday, negotiators are far from finalizing a new trilateral trade agreement.
He did not respond directly when asked if Canada needs an "ironclad commitment" that Trump will not impose the tariffs.
The US trade representative, Robert Lighthizer - Freeland's counterpart at the talks - complained on Tuesday that Canada was not making enough concessions and warned that time was running out.
"I don't like playing chicken with the future of the Canadian economy", the ambassador said at an event hosted by Politico. "Frankly, we're thinking of just taxing cars that come in from Canada".
Negotiations for possible exemptions for Mexico and Canada from USA steel and aluminum tariffs imposed earlier this year will take place after the new Nafta deal is agreed on, Lighthizer said.
"I think everybody knows what each other's position is on all of the major issues and I think it's really a question of whether or not the US wants to have a deal".
Any final deal on NAFTA will include language around gender and indigenous rights, as initially sought by Canada, he said.
Trump has previously floated the idea of striking two separate bilateral trade deals with the NAFTA partners. "We really don't like their representative", he said.
"Those days are over", said Trump, who didn't specifically mention Canada in his speech. If Canada comes along now, that would be best.
He was also asked if Ottawa needed a solid commitment from Washington that it won't make good on its threats to impose duties or tariffs on Canadian auto exports.
Freeland declined to comment specifically on Chapter 19, saying only that "rule of law is an extremely important part of how we do things, including trade".