Warning! The following contains spoilers from chapter 284 of My Hero Academia. You have been warned.
It looks like round one of Deku’s and Shigaraki’s grudge match is about to reach its climax. Deku is really pushing himself harder than he has so far. He is using THREE quirks at the same time in a desperate attempt to bring Shigaraki down: Blackwhip, Float, and the base power of One For All. It is clear that using this many quirks simultaneously is putting a serious strain on Deku because his control over Blackwhip is starting to wane. It was revealed that Deku’s left arm was broken when he attempted to knock Shigaraki out with 100 % of One For All. Even so, Deku is not letting that stop him. He is ignoring the consequences using One For All at full power would have on his body and is mindful only of his duty as a superhero.
Deku is not holding back this time. He is so desperate to bring down Shigaraki that he is using 100% of One For All a lot more frequently than normal. It is like when he defeated Overhaul except Eri is not here to heal his wounds. Because of this, Deku is nowhere near ready to unleash that much power by himself. At this stage, Deku can only unleash 30% safely and 45 % of One For All in short bursts without damaging his body. Now, by using 100% of One For All so many times, Deku is drastically increasing the damage that will be done to his body. With his left arm already broken, Deku also struck Shigaraki with his right leg and right arm. By the time he is done pummeling Shigaraki, Deku will only have his left leg intact. However, it is possible that the extreme kickback will cause a lot more damage to Deku’s body than just a couple of broken limbs. My guess is that Deku’s skeleton, muscles, and even his organs will be so grossly crippled that he would be out of commission for the foreseeable future. I doubt even Recovery Girl would be able to undo this level of damage. In order for Deku to get back on his feet, Eri would have to use her Rewind quirk to heal his wounds like she did during the battle with Overhaul.
Shigaraki, on the other hand, is in a different position than Deku. Because of his incomplete surgery, Shigaraki’s regeneration has slowed down. However, even though his healing process has slowed down, Shigaraki can still heal his body. That is an advantage Deku does not have. Also, before Shigaraki’s body started breaking down, his regeneration was working at full power when fighting Endeavor and when Aizawa blinked. With this in mind, Shigaraki’s regeneration may not stay this slow for the remainder of the fight. Instead, I think his healing speed will fluctuate from being fast to being slow. This fluctuation would fit well with Shigaraki’s unstable body. In addition, Shigaraki’s body has already proven to be able to withstand 100% of One For All’s power. Also, Shigaraki has who knows how many quirks stored within him thanks to the All For One quirk.
Overall, this first round between Deku and Shigaraki will result in both characters having their bodies broken. However, Shigaraki can heal his body and Deku cannot. Due to this, Shigaraki can recover on his own while Deku will likely be immensely crippled for a while. With Deku in such a vulnerable state, Shigaraki will probably seize his chance to steal One For All from Deku. Having his body damaged is bad enough, but having his quirk stolen as well would be soul crushing for Deku. In addition to healing Deku’s body, Eri’s Rewind may restore One For All to Deku. Only time will tell as the story progresses.