Because I wanted one but you decided not to stick around long enough for Tsunade to replace yours!

[Clearly someone has told Sasuke that they both lost an arm in their fight. Sakura didn't know that Naruto had gotten that replacement but Boruto knew so Sarada had to know too. But hey, if Sasuke wants proof then he can have it - up close proof.

His left hand reaches into the pouch at his back and Naruto uses a shuriken to knock the next wave away instead of attempting to dodge it. He keeps heading straight for Sasuke, intending to use his own fist to knock some sense into the guy. It really isn't the fight of two years ago when they came at each other with everything they had, exhausted themselves until they had no choice left but to use their fists. Fighting until they're black and blue, unable to move and bleeding out.]

If fighting until we both lose an arm is what it takes to make you realize that I'm not giving up on you, then that's what I'll do. Again if I have to.

[Naruto closes the distance and aims his fist to punch Sasuke directly in the face. That's it, a simple punch. Well, an extra-extra strenght punch with Kurama's enfused chakra but still.]

I'll keep going until I can't and then I'll get patched up and do it all over again.