Well he could go into Sage Mode at any time now can't he? He's not in battle, and he's only just tired. As long as he can gather the chakra, and if he's not so exhausted that he can still move around fast, then it should be possible.

Speaking of Sage Mode; It's pretty smart of him to enter Sage Mode when going to talk to Pain. He doesn't know what Pain might try, and doing this is one step towards being prepared for anything. A very wise choice.

EDIT: poop beat me to it.