"Mosul is supposed to be easier than these other cities outside Mosul, which we've been liberating -- because these are the outskirts," Abadi told CNN's Christiane Amanpour on Wednesday in New York, where he was attending the United Nations General Assembly.
"They're supposed to be more pro-Daesh than the city itself," he added, using another name for ISIS.
The Iraqi military has been slowly recapturing towns surrounding the city, with the support of US airstrikes; most recently, the town of Shirqat.
Surrounding the city will prevent ISIS from supporting and resupplying Mosul, a US defense official said.
Nonetheless, Abadi said, his government is preparing for the worst.
"We are planning for a fight for many months," he said. "But we anticipate the fight for Mosul will be easier than probably Ramadi."
All momentum in the anti-ISIS fight has been building toward Mosul, which ISIS captured in dramatic and rapid fashion in June 2014.
The assault could begin as early as October, according to several US officials.
Iraqi security forces dropped thousands of leaflets over the region south of the city this past weekend, warning citizens in anticipation of a new offensive.
"Protect yourself, don't be human shields for the enemy, leave the town immediately," said a leaflet shown by the Iraqi military to CNN.
"We have a schedule," Abadi said. "And this schedule is on time. And we are progressing very rapidly."
"There is a certain time, which has been fixed, but still I have to decide the last minute when our forces will start the offensive to liberate Mosul."
That decision, he said, will depend on "last-minute" military preparations and developing appropriate plans to protect civilians in the city.