I remember seeing a documentary on Johnson where they discussed the background of this fight. Interesting stuff, that is, if you're a boxing afficionado.
Hockey's another brutal sport as far as teeth goes. In the old days I'd say about 90% of the players lost some of their teeth, now with less fighting. it's probably around 50%. Certainly another who gets hit in the mouth with a slapshot is going to loss teeth, mouthguard or not. As far as boxers go, there was one recent fight with Gatti that I believe he had a tooth knocked out. And I have to Ketchel - Johnson fight on video but you can't see any teeth being knocked out, it certainly happened given how Ketchel was just floored with that punch, which I believe were delivered with 4 oz gloves. I too have that documentary, bought it on PBS. Unforgiveable Blackness.
I'll call bull**** unless he was either fighting a 14 month old baby, or a chap by the name of "Gummy Joe".
Years ago in hockey, Wayne Cashman caught Barry Wilkens in the mouth with a puck. Wilkens bent over, spit out at least three teeth. Goes to the bench comes back out with gauze in his mouth and finished the game. Next to boxing, hockey players are right there in the toughness department...