Board Thread:Versus Debates/@comment-43403813-20200102180859/@comment-9782468-20200104055542

Nagato's a sensor, and Obito using Kamui on himself takes longer than using it on people, leaving Nagato ample time to sense and react to this kind of attack.

In fact, he'd already know something was up as soon as he sensed Obito's chakra had completely disappeared out of nowhere.