A change in: Mentor
Because Shisui was just as good as Itachi.
Sasuke begged his brother to train him every day, only to be sent off with a forehead poke. Today Sasuke decided to try something new.
"Train me, Shisui-nii-san!" Shisui smiled.
"Come on, Sasuke-chan! If you can learn the Body Flicker, even just a little, in three days, you're going to be my new favorite cousin!"
Haku was fast with those mirrors, but, unfortunately for him, Sasuke was faster. Sharingan spinning, Sasuke moved to intercept Haku at every opportunity. Naruto may have broken the mirrors, but without Sasuke's speed, they both could have been injured or worse.
Orochimaru moved to give Sasuke his curse seal. Abruptly, Sasuke was no longer tied to the tree. Even the eyes of one of the Sannin couldn't keep up with the speedster. Helpless, Orochimaru fled.
Sasuke donned Shisui's old ANBU mask. He would keep the memory of his elder cousin alive in both technique and name. Shunshin no Sasuke met his brother as he lay dying. Itachi saw with what little vision he had left, an image so similar to Shisui's after-image clone. In his mind, the images of Sasuke and Shisui overlapped. Itachi died with a smile.