The professor of non-profit marketing at Bristol Business School made the comments at the Action Planning Funding the Future conference in London last week.
Presenting his 'Strategies for Meeting the Fundraising Challenge', Sargeant said fundraisers should rethink payroll giving.
"Payroll giving was invented at a time when it was genuinely good to give through the payroll," he said. "But now, with direct debit, things have changed."
He argued that people should give by direct debit and, rather than using payroll giving, charities should recruit people in workplaces to sign up their colleagues to give.
He stressed that 'give as you earn' schemes already launched by charities were not a waste of time, but added that the beauty of direct debit donations was that employees could continue them even if they changed jobs.
Sargeant added: "I'm not trying to ditch payroll giving, but to stimulate debate."
Cathy Pharoah, research director at the Charities Aid Foundation, was also on the panel of speakers, but she rejected the idea of getting rid of payroll giving.
Instead, she argued that payroll giving had grown steadily and that the most important thing was to get more employers on board.
At the conference, which was held in association with the chief executives body Acevo, Sargeant asserted that "now is a bad time to be a fundraiser".
Although there has never been a specific incident to threaten public confidence in England, Sargeant said fundraisers "can't afford to be complacent".
He advised charities to engage in genuine recruitment, rather than rely on swapping databases of donors.
"We can do a lot more to keep donors loyal," Sargeant said. He advised people to "watch the door", because donor retention was more important than donor acquisition.
He also urged charities to "seek multiple engagements", particularly among the young, where those who campaign or volunteer might be more inclined to give.