Previous reports had pegged two new phones - one 4.7-inch model and one 5.5-inch model - for launch in September, but Kuo says that Apple is having issues with the iPhone 6's touch panel, as well as color uneveness on the metal casing.
"These problems will likely be even more complicated with a larger size," he said.
AppleInsider's Neil Hughes notes that Kuo has a "respectable track record" when it comes to Apple's plans, though his word is still no guarantee.
Kuo also said that he doesn't expect that the iPhone's new, extra-durable sapphire glass front panel will "easily pass the drop test near term" for the 5.5-inch phone.
If the phablet model does launch this year, Kuo believes it won't happen until after mid-fourth quarter, later than the 4.7-inch iPhone 6.