Board Thread:Naruto Discussions/@comment-45851667-20200520192705/@comment-32685487-20200520205959

Shisui using his Mangekyo once every 10 years is full on bull, that line is clearly referring to the fact that the eyeball is inside the crow, context is extremely important here. Also Shisui himself already said Itachi would surpass him(He said this before he died) but what Shisui didn't know is that Itachi would gain a terminal illness a few years later and therefor have to take medication just to stay alive.