But, asked about the repeated suggestions made by supporters of Mr Freer, that Ms Berger was wrong to stand in the seat, he launched an outspoken attack.
"I think it is insulting and disgusting the way Luciana has been treated by some people," Mr Umunna told the JC.
"She has stood up for the community, and has taken a lot of flack and that is an understatement. She has had her life threatened for the simple reason of standing up for the community.
"For that to be brushed aside, and for there to be any suggestion whatsoever that she would ever facilitate Jeremy Corbyn becoming prime minster is not only unfair, it is deeply unjust."
He recalled the treatment dished out to Ms Berger before she and he quit the Labour in February, adding: "One of the reasons I left was that I could not bear to see the way she was hounded out of the party by racist supporters of Jeremy Corbyn."
Mr Umunna also gave short shrift to the suggestion Conservative leader Boris Johnson represented a better option as the country's next Prime Minister.
"I don't think it is in anybody's interests to give a blank cheque to a man who is the in the slipstream of right wing authoritarian nationalists," he said.
"For me as somebody who is a member of a minority community - that makes me shudder."
During his speech, Mr Umunna had said the UK risks becoming a "vassal state" of the US if Mr Johnson gets a majority in next month's election.
He warned that the country could become "Trump's poodle" in the speech ahead of next week's NATO leaders summit in London.
Mr Umunna said he had chosen to focus his speech on Mr Johnson and not the threat also posed by Mr Corbyn's views on NATO "because at the moment it is quite clear as the polls stand halfway through the campaign that there are two scenarios - either Boris Johnson gets a majority or he doesn't."
He told the JC: "I don't think anyone can accuse me of not pulling my punches when it comes to calling out Jeremy Corbyn's lamentable record when it comes to NATO.
"He has opposed the liberal international rules-based order frequently and he has often sided with the enemies of our country.
"People who are hostile to British values. I think that is one of the reasons perhaps that Labour have struggled at this general election."
Asked about the politics of Gordon Nardell, his Labour rival in the election and a close friend of Mr Corbyn, Mr Umunna said: "The politics of which Jeremy Corbyn's candidate in the City of Westminster is a politics which the overwhelming majority of people in the area reject - and strongly.
"This is an area that includes the City, Knightsbridge, Belgravia, Mayfair, and St James and is never going to elect Jeremy Corbyn's MP with that type of hard-left politics.
"Whereas these areas are putting their faith in my party because they are liberal, internationalist progressive and they know we are pro-enterprise too.
"I think the hard-left background of the Labour candidate is a problem - but actually the bigger problem is he is sponsoring somebody to become PM who the majority think will destroy the businesses that are situated in our constituency."
Mr Nardell, a QC, was selected to fight the seat after a stint overseeing Labour's disciplinary investigation process into antisemitism.
But he has a history of hard-left political activism and opposed the full International Holocaust Remembrance Alliance (IHRA) definition of antisemitism, which the party resisted adopting last summer but ultimately did after a protacted row with the Jewish community.
Mr Umunna said he was confident that the Lib Dems could win "in a number of Tory held seats where we are now clearly a strong second".
"Tactical voting will win those seats," he said.
