How did Madara implant his eyes in Nagato without Nagato noticing it?

[Q of the Sharingan]

There's one thing that's been bugging me but how did Madara who was a old man and was connected to the Gedo Mazou statue implant the Rinnegan into Nagato without him finding out about it?There's only one way I can think of that explains how Madara was able to pull it off and that is he had Zetsu kidnap Nagato while putting him in a genjutsu.I mean it makes the most sense because Nagato would be wondering what happened to his eyes otherwise,but if he was placed in a genjutsu while Madara did the transplant and then wiped his memory afterwards then Nagato would be none the wiser.So how do you think Madara implanted the Rinnegan into Nagato?I would like to hear everyone's opinion

He could have allowed it to "awaken" with some kind of time released method that was similar to what Itachi did to Sasuke to use Ama on Tobi, but more permanent.

 

[Untung]

Just like my question, if nagato is uzumaki.. Then his parents is uzumaki to, but how easy his parents dead from random kohona shinobi??? This is just Secret no Jutsu from khisi

 

[SIR HERDERP PRESIDERP SDO]

Maybe Zetsu implanted it on him when Nagato was asleep

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[Vexion]

Its a mystery to us all