In the Boruto series of Naruto, it is hinted that Uchiha Itachi's illness might be a result of a serious injury rather than an actual illness. In Boruto, the next generation of the Naruto universe, the Otsutsuki clan comes to Earth to collect the chakra of the tailed beasts. This is when they encounter the seventh Hokage Naruto, who is at the peak of his power. 

One of the Otsutsuki members, Urashiki, plans to time-travel back to Naruto's childhood to steal the Nine-Tails chakra.

In this past timeline, Naruto is still a low-ranking ninja and cannot help Sasuke fight against Urashiki. Sasuke might seek help from his older brother, Itachi, who was still a member of the Akatsuki at that time. With Itachi's assistance, Sasuke and Itachi would have the power to defeat Urashiki.

Itachi likely got injured while protecting Sasuke during the battle. Sasuke, knowing the consequences of altering the past, would keep this event a secret between him and Itachi.