Countering a headbutt is probably easier said than done considering how close the head is and how long mashiba's arms are. Not to mention the super awkward way youd have to position your fist, especially with a southpaw.
Not feeling this one bois. I've been thinking this since a while ago; Shiba-kun is either gonna end up crippled or dead setting up Ippo's return and eventual rematch with te quiero over here.