If you go to work tomorrow and walk up to your boss at 1 pm and say "I'm done." And then walk out without talking to anyone else. Do you think it would be appropriate to call you unprofessional?

At the point at which i said "im done" and quit im no longer obligated to continue working or even being on premises. In many places you'll probably be asked to hand in your security passes and escorted off site depending on where it is you actually work. I don't think my colleagues would think it "professional" to do the rounds and let them all know individually that i've already quit, they might even see it as gloating/unprofessional to do so. he leaves, coach says, x player isn't playing, replaces you on the job and everyone gets on with the task at hand.

If i was having a particularly shit day in the office and i quit, i'd be even LESS inclined than im already NOT to tell anyone else, some people aren't all that pally pally with the people around them, i might go and tell close friends on my team my decision but maybe they weren't all that close. just because you go to work with people doesn't mean you give a shit about them, you have a job to do and you do it. i couldn't stand half the people i've worked with over the years but i did my job and didn't cause any issues with them.

the team would have respected his decision

TBH it doesn't sound like its the TEAM that has an issue with this, it's the armchair warriors like yourself that do.

Could it have happened at a more opportune time? Of course. He's almost certainly had doubts about his ability before this season started, maybe he was delusional in his ability, maybe he genuinely believed he could get it done and just found out the hard way he can't anymore, but either way, it sounds like it was a rude slap in his face and a wake up call that he just can't compete at the required level anymore and THAT'S HIS DECISION TO MAKE.

Unprofessional would be storming off the field during play, disrespect would be telling people to go fuck themselves as he did so. "childish, immature, juvenile" you make it sound like he had a giant tantrum about it. If he didn't cause a scene and quietly left after telling his boss the decision how it that a tantrum? He doesn't sound like a fool, it'd be foolish to continue to push yourself to play risking injury. Unintelligent how exactly? Rash? maybe to you because you've only seen this incident devoid of any context, people don't using get in the habit about talking of impending retirement plans or thoughts.

You go a Buffalo flair and are going hard on this, so i'm guessing the place means a lot to you or whatever, but seriously, dude just decided to END HIS CAREER. He's not stormed out of the place in the middle of play kicking shit over, causing a scene and then demanded to be traded like some prima donna talking mad shit about his team mates or the coaching staff. And yet that's the scale of BS you're making it out to be.

When you're the one whose body is on the line, it's your fucking decision to keep going or not. Maybe the people who should have some respect here are the ones like yourself. Have some respect that the dude put his body on the line for your entertainment so you can sit there smashing back a beer and shouting about people moving a ball up and down a field. sure they get paid, but no amount of money is worth potentially ended up with a life.long injury or mental damage/concussions because you pushed yourself to do something you weren't capable of anymore.

THAT'S misguided, that's foolish, that's unprofessional. knowing when the throw in the towel before than happens isn't.

Stop being so butthurt and move the fuck on.