De Niro made his comments at a groundbreaking for a Nobu hotel in Toronto.
Trudeau has not issued any public remarks about Trump's latest attacks.
While it was hoped that the trip could be used to help defuse tensions over a brewing trade war between the two countries, Trump began firing off tweets lashing out at Trudeau nearly as soon as he left.More news: United Kingdom minister resigns, plans to rebel over parliament's Brexit role
"I mean, this type of language is accepted in Trump's America", Seawright claimed.
"Mr. De Niro's comments were unscripted and unexpected".
The president called Trudeau "disgusting" and "weak" for how he pushed back against steep tariffs imposed by the USA on steel and aluminum imports from Canada and other trade allies.More news: President Trump fires back at Robert De Niro after Tony Awards rant
PM Justin Trudeau of Canada acted so meek and mild during our @G7 meetings only to give a news conference after I left saying that, "US Tariffs were kind of insulting" and he "will not be pushed around". The exclusive concert residency, "Springsteen on Broadway" - or as De Niro referred to it, "Jersey Boy" - features the Boss performing his music and sharing stories from his 2016 autobiography "Born to Run". During the opening lunch at the Tribeca Film Festival in April, the actor called Trump a "mad man" and a "low life".
"Bruce, you can rock the house like nobody else", he said.
American viewers watching the Tonys at home only heard silence - the broadcast on CBS bleeped De Niro's expletive-heavy intro - but they saw the audience's largely enthusiastic reaction, the Associated Press reported.More news: Rose McGowan indicted on cocaine charge in Loudoun Co