I am sorry, man.I dont read manga.pls give me a scan where itachi told nagato not to underestimate nagato.pls man.
I'm not sure what scan it is but during the first shippuden arc when Gaara was taken by Akatasuki and team Kakashi and team Gai were on their way zetsu tracked them using the mayflower technique. They were sealing Gaara in the statue atm and Zetsu told Pain (Nagato):
Zetsu: ''A very troublesome shinobi is on the way''
Nagato: ''Who?''
Zetsu: ''His name...... is Might Gai''
Nagato: ''Might Gai!? Who is he?''
Itachi: ''An elite leaf Jounin, a master of taijutsu......don't underestimate him......you'll be killed''
Thats when Nagato used that body double jutsu to send fake Itachi and Kisame out to stall them.
Pretty sure it went like that.