Question. Did the original Japanese actually use the terminology of Tommy John regarding the surgery? I ask because if so, that means that Tommy John is officially canonical character in the major universe lol. I will say it’s not entirely realistic for middle schoolers to have it because most middle schoolers won’t break that elbow at such a young age mainly because the coaches don’t make them throw that hard and most middle school don’t have bodies that can anyways. This plot point could’ve been more realistic if it was high school but because we are still in middle school, it’s a weird thing to put in.
It says Tommy John. It's not strange to mention Tommy John, because his name has become bigger than him as a player, and because Mitsuda often uses real life professional players(it was more common in the original work). On the little league edition of the original MAJOR manga, the players from the Yokohama and Tokyo Giants teams were all based on the real players from their rosters(Makihara Hiromi, Hideki Matsui, etc. I have made a more comprehensive list than this, but I don't have it in hand so forgive me), in the All-Star game when Gibson invites Goro to America, all players were based off the 1995 MLB All-Star game roster. Here is an article with the rosters, you can compare them to those in the manga, it is pretty obvious:
https://en.m.wikipedia.org/wiki/1995_Major_League_Baseball_All-Star_Game
There were also players like Jack McDowell and Ken Griffey Jr. who were featured on the little league edition.
A lot of players on the WBC edition were also modelled after real players, here is an article on Japanese:
https://baseball-89.com/major-modelplayer/
In 2nd it has become more rare, but there is a moment where you can see Chiba has a Shohei Ohtani poster in his room.
I think all of these players count as "canon". Rather, the "major universe" is just the real world.
I also think having Furuya undergo Tommy John is silly and unrealistic because he is so young, even players who are known fire throwers often have the surgery on high school, college, or once they have begun their professional careers. Mitsuda often relies on arm injuries for plot, he has become very bad at coming up with original ideas. On the words of anonymous 5chan user, "these guys always break their elbows", lol.
I'm sorry because I wrote a long reply.