McGregor was answering questions on the UFC 202 media conference call Friday -- when he admitted that he underestimated Nate's chin and toughness the first time around ... but he won't make that mistake again.
"He weathered the storm," Conor said ... "I'm just gonna be a lot more prepared for a man that can stay in there with me."
That being said ... Conor said confidently -- "I will K.O. him inside the 2nd round."
Funny moment though ... Conor says Nate was a "heavy bag with eyeballs" during the first round and a half of their first fight.