Board Thread:Versus Debates/@comment-31337598-20180326183459/@comment-26107879-20180329214426

"Later, when Orochimaru saw his opportunity to steal his body to gain possession of the Sharingan, Itachi used his Sharingan to stop him from doing so, while also cutting Orochimaru's left hand off to prevent him from breaking the genjutsu he was under, which lead to him leaving Akatsuki to acquire a new body.[26]"

It's known that Itachi could place him in a Genjutsu and prevent him from breaking free. It wasn't just "Give me your body." and a "No." Kind of ordeal. It was a small battle that Itachi easily won at the time. Itachi damaged his body to the point he needed a new one.