The punch to Michael's facial area broke his cheek bone which gave him a long-lasting black eye (and brought on his sinuses to continually run - that's why using a handkerchief all the time) until eventually he bought again to The us and experienced surgery to fix it (Freddie states, https://sitesrow.com/story8594586/considerations-to-know-about-7-reason-why-the-godfather-master-piece