User blog comment:TwixBreak/Assassin's Creed 5?/@comment-5546424-20131112151901/@comment-3046679-20131112155704

Theory - the knife was coated with a debilitating agent that wears off very, very slowly, and for plot convenience apparently after each assassination. Al does mention that he remembers how to pickpocket, eavesdrop and shit. It's just his combat that was impaired.

Fact - knife was infused with Space Magic from ME3.