"It's a fair bet to say that half of all New Yorkers, and maybe more than half, will end up contracting this disease and that's worrisome, very deeply worrisome for all of us," he said on Wednesday evening while at the same time in Washington the White House Coronavirus Task Force said that the number of new cases there were relatively "constant" in a sign the social distancing norms were working.
De Blasio's claim about the city a total population of 8.6 million was also in contrast to New York State Governor Andrew Cuomo's statistics that the rate of increase in cases was showing signs of slowing.
Cuomo said at a news conference earlier in the day that rate of hospitalisation of coronavirus patients which was doubling every two days on Sunday, had slowed to doubling every 3.4 days on Monday and 4.7 days on Tuesday.
New York state is under a semi-lockdown with a total closure of all non-essential businesses and offices, and Stay-at-Home orders for the people.
New York City, the epicentre of the pandemic in the US, had recorded 17,856 cases on Wednesday with 192 deaths.
New York is reporting an infection rate of 1 in 1,000 compared with between 0.1 and 0.2 per 1,000 in other states, according to Vice President Mike Pence.
The White House coronavirus co-ordinator, Deborah Birx, said at the task force brifing, "You will see in New York City, the number of (new) cases per day has been relatively constant over the last three days.
Birx said that these numbers are clearer to interpret now because the testing backlog has been reduced significantly.
However, she said the hospital cases in New York will continue to rise, although not any more than a "consistent day over day rise."
This increase in hospital cases are a reflection of the infection rate before the full mitigation efforts were put into place.
Birx warned against complacency and urged Americans "not to let up for a moment" as the 15-day mark nears for the first test case of social distancing guidelines.
"This is a real call to every person in New York City in the New York metro area to continue to follow the White House social distancing guidelines", she said.
Bix said that 56 per cent of new cases are still coming from the New York metro area that includes a few counties in New Jersey state.
New York City's Health Commissioner Oxiris Barbo said that the percentage of people in New York City infected with COVID-19 will "likely 50 per cent but it could also be much higher" by September.
"I think it's safe to say that there are tens of thousands of New Yorkers that are somewhere along the spectrum of having COVID-19 or having been exposed to COVID-19 and that's why it's so important for people to stay home," she said.
De Blasio, who has criticised President Donald Trump's handling of the crisis, reiterated his demand to call out the military to help him deal with the city's coronavirus pandemic.
The Trump task force is urging anyone who has been in the New York area to self-quarantine for 14 days to ensure that the virus does not find new seeding spots in the community.
Florida became the first state to require people coming from New York to self-quarantine for 14 days. Governor Ron DeSantis said that anyone flying in from the city and surrounding areas will face criminal penalties if they do not self-quarantine.
With its warmer climate, Florida is a favourite retirement spot for people from the colder northern states and 20 per cent of its population is over 65 - the age group most susceptible to COVID-19 - compared to 16 per cent nationally.
New York currently accounts for 31 per cent of people succumbing to COVID-19, according to data from Birx and New York Governor Cuomo.
(Arul Louis can be contacted at email@example.com and followed on Twitter @arulouis, and Nikhila Natarajan can be contacted at @ByNikNat)