he said that there is no need for Naruto to remember such complex seals...
hand seals are for modyfing chackra, both the quantity and the type, the rasengan doesn't base itself on chackra types or insanly huge amounts of chackra, it's all about the spinning motion...
so, like most other jutsus, it's possible to be done without using handseals, it's just harder...
and as for why Jiraya had him learn it without the seals... it was probably so that Itachi won't be able to copy it....