Angela Rayner, who served as shadow education secretary for much of Corbyn's time as leader, was asked by Times Radio whether she could hand on heart say that Corbyn would have done a better job than Johnson and she responded with: "100% yes."
The government's handling of the pandemic has been brought into question after Johnson's former top aide Dominic Cummings made a series of incendiary remarks regarding the behaviour and capability of the people in charge of the country at a marathon select committee meeting on Wednesday.
Cummings told the committee tens of thousands of people died unnecessarily because of the government’s failings over coronavirus and Johnson was “unfit for the job” of prime minister.
Watch: Five key moments from Cummings's testimony
The former aide was critical of several leading Tories but he also hinted he didn't believe Corbyn's Labour Party would have done better.
He said the country faced a "very profound question" about the political system which decides our leaders.
He said: "Any system that leaves people with the choice between Boris Johnson and Jeremy Corbyn is obviously a system that's gone extremely badly wrong.
"There are thousands of people in this country who could offer better leadership than those two."
A YouGov survey in March did not agree with Rayner's comments, with most saying Corbyn would have either done the same or worse at handling the pandemic.
Only 8% of people believed a Corbyn-led government would have managed to roll out the vaccine faster than the current administration.
This rises to 14% when it came to stopping the spread of COVID-19 but a whopping 50% of respondents said a Corbyn government would have done worse at handling the economic impact of the pandemic.
The current Labour leader Sir Keir Starmer only fared slightly better when asked if a government led by him would have handled the pandemic better than Johnson.
Cummings criticism has been denied by Johnson saying “some of the commentary I have heard doesn’t bear any relation to reality”.
However, he repeatedly dodged giving full answers when asked about direct points made by Cummings at the committee.
Apart from his damning assessment of Johnson, Cummings saved his fiercest criticism for Matt Hancock over failings around care homes policy, personal protective equipment procurement and his public pledge on a testing target which caused disruption in Whitehall.
He claimed Hancock should have been sacked 15-20 different times for his behaviour and said the health secretary regularly lied at meetings.
Watch: PM did say 'let the bodies pile high', says Cummings