Board Thread:Naruto Discussions/@comment-26403664-20160228225013/@comment-26369387-20160315125921

Fact 1: One needs both Mangekyō Sharingan to awake Susanoo.

Fact 2: To get EMS, two Uchiha relatives needs to switch their eyes between them(siblings most common).

Fact 3: Six Paths Chakra enhances ninjutsu(which Susanoo is) and with reincarnations of Indra it awakens Rinnegan.

Fact 4: People that have shown to have PS(Madara, Sasuke, Obito through Kakashi) had EMS and/or Six Paths Chakra.

Fact 5: Edo!Itachi, thanks to Kabuto's improving the technique, was brought very close to his original strength when he was alive. Maybe not 100%, but still very close.

Fact 6: Itachi could use MS without damaging himself, similiar to how EMS doesn't damage Sasuke.

Fact 7: Itachi's best form of Susanoo ever shown is half-body armoured Susanoo.

Fact 8: Indra is direct son of Hagoromo, inheriting his powerful Eye Prowess, chakra and spiritual energy.

Fact 9: Dozens of generations passes between Indra and Madara/Sasuke, weakening connection to Hagoromo.

From here we can conclude that Itachi without EMS or Six Paths Chakra cannot use PS.

Heck, when you really think about it, Kishimoto could have given Itachi EMS by having Kabuto to implant Sasuke's original eyes to Edo!Itachi.