"I’m sorry but who ever signed off Keir Starmer being subjected to 5 mins of why Brian Cox will never vote Labour again is an evil genius," Ava Santina said.
Keir Starmer has tested positive for Covid-19 - and will not be taking PMQs or responding to the Budget.
Amnesty International has said the takeover is "an extremely bitter blow for human rights defenders".
The Labour leader said Johnson has no plan to shift Britain to a high-wage, high-skill economy.
Earlier in the week The Sun had criticised Starmer, saying his anti-Brexit and pro-immigration stance would never get Labour back into power.
Sir Keir said he was pleased to have "got my own house in order" at the party's Brighton conference.
Starmer’s performance went down better than either Jeremy Corbyn or Boris Johnson’s first conference speeches, polling found.
Thin on policy but high on emotion, the Labour leader's 90-minute speech outlined his priorities for government.
Labour leader Sir Keir Starmer will address activists in Brighton as he seeks to persuade voters that his party is a government-in-waiting.