I agree with most of the above posts.
I feel it's more media driven term,picked up by fans,and used over,and over. Even the Best in the world didn't climb to the top by themselves.The best race car driver in the world is only as good as his vehicle he's racing in.Boxing,you have trainers,and corner,and a cut men. Even in professional golf,you have caddies,and swing guys.
I'm not sure it's a talent, leadership thing,a stats,or money thing.
For me, I have to blab a bit about the Cubs here,because I see only missed a couple of games all last year.
We had many different guy's be the "Man" throughout the year. Jake Arrieta had an historic
second half of the season pitching,but Jason Hammel had good
1st half of the season before getting hurt,and struggling through the second half. Early in the Season Joe madden said to Dexter Fowler "As you go,We Go" He had period where for like 2-3 weeks the ball was a beach ball.Kyle Schwarber in Cleveland was called up the be the DH when the offense was struggling,Kris Bryant,Chris Coghlan stepped up at times,Rizzo was good most of the the year.
I feel like in a team setting there needs to be
feeling that they have those few guy's that are the "guy" are the man.They know everyone is important to the success of the team,the "guy",the "Man" can only do his thing with the others help. (I'm not sure if I'm saying that right?

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