Farage was speaking on his new LBC radio show just one day after the party elected Diane James as its new leader.
He accused Carswell of "sniping from the sidelines" to give the "impression of division" in the party, adding:
I do think that Douglas Carswell needs to make a big decision. Is he going to support Diane James, is he going to support the party's aims. It he going to go on sniping from the sidelines.
It's no good for Ukip, and I don't think looking at him, and he doesn't look very happy, I can't believe it's very good for him either.
Carswell and Farage have been at loggerheads for years. Carswell has only been given a five-minute speaking slot in the final hours of the Ukip party conference.
Farage gave a speech to a packed hall of Ukip supporters yesterday, who applauded him for his time at the head of the party. He warned that Ukip must remain a strong force in British politics to make sure Prime Minister Theresa May doesn't opt for a "soft" Brexit.
New leader Diane James echoed his concern when she gave her acceptance speech later in the afternoon, and accused the Prime Minister - "Magpie May" - of stealing Ukip's policies, namely Ukip's call to bring back grammar schools.
UKIP is the opposition party in waiting! pic.twitter.com/7lQWfNMMUs— Diane James MEP (@DianeJamesMEP) September 16, 2016
James said Ukip was "the opposition party in waiting" and called on May to trigger Article 50 - which would kick-off Brexit negotiations - as quickly as possible.
"So, Mrs May, you’re now looking, and if you’re watching TV this afternoon you’ll be watching the opposition party in waiting," she said. "Stop the faff, stop the farce and stop the fudge and get on with it. Invoke Article 50."
Farage may have left the top job at Ukip, but he is still the centre of attention. Late last night, it emerged Farage had a naked swim off the Bournemouth Pier to celebrate the end of his leadership.
Businessman Arron Banks told the BBC:
We had dinner last night, a few drinks, it was late at night and we went skinny dipping off the Bournemouth Pier.
If ever a man's about to leave British politics, that's it.
Labour MP Angela Eagle, who was on the same show, said there "should be a law against it". It is not clear whether she meant all skinny dipping should be illegal, or if there should be an exclusive ban for Farage.