Scottish Tory leader Douglas Ross said on Monday he did not expect the Prime Minister to come to Scotland to campaign for his party, after Mr Johnson had said "wild horses" could not keep him away.
The Prime Minister is considered to be unpopular in Scotland, with a recent poll by Savanta ComRes putting his net favourability rating at -21 per cent.
When asked about another visit from his party’s UK leader – the third in recent months – Mr Sarwar said: "Keir Starmer has been up in the campaign, I think you can see who's an asset and who's not.”
Referencing Mr Johnson’s previous remarks that devolution had been a “disaster”, Mr Sarwar added: "I can understand why Boris Johnson doesn't want to visit – Boris Johnson is a disaster, devolution is not a disaster.
"Boris Johnson is the biggest threat to the United Kingdom, Boris Johnson has delivered us Brexit and things are so bad that even Ruth Davidson has walked away."
Despite the attack on the Prime Minister, Mr Sarwar said his party were focused on a "positive campaign", adding: "That campaign in Scotland is being led by me.
"I'm in charge of what we do in Scotland, I'm in charge of our campaign in Scotland, I'm in charge of working with our members to have the ideas across Scotland to deliver that national recovery."
When pushed on whether Sir Keir would return to Scotland, he said: "We'll see – I believe he is going to come back up.”
SNP deputy leader Keith Brown said he believed Douglas Ross is “running scared of letting Boris Johnson campaign in Scotland.”
While Mr Ross has said the Covid pandemic was causing restrictions on travel and Mr Johnson was focused on the vaccine programme, the Prime Minister did visit Scotland in January when coronavirus levels were much higher.
Mr Brown said: “Douglas Ross is embarrassed by his boss. Boris Johnson is such a liability in Scotland that the Tories are desperate to keep him out of the country."