observation

[hogoromo]

sasuke also have a bandaged head while naruto has a bandaged hand in the sketch of the LAST:sasuke::naruto:

 

[Omegga Shenron]

maybe naruto sealed his head or something getting rid of indras chakara

 

[SuperChief]

Naruto might have scratched Sasuke's forehead and damaged his hand in the process.

 

[hogoromo]

or it might be bacause of the combination of chidori and rasengan

 

[BenjerminGaye]

Naruto beat him so hard he lapsed sanity and returned to leaf.

 

[Vapid]

He Chidori'd himself in the skull to avoid Naruto's TnJ.

 

[Sakura x Ino]

Sakura surprised him by coming out of nowhere and shanaroing him in the forehead because of how he had gave her hope of a better future because of his less hostile actions, and then crushed it by turning evil again