BJ Penn should hang 'em up 'cause he's a shell of the dominant fighter he once was ... so says former UFC champion Frankie Edgar.
Penn, 38 years old, returned to the Octagon Sunday night after a 3 year layoff ... and was absolutely DESTROYED by up-and-coming star Yair Rodriguez ... igniting the debate, should BJ retire?
TMZ Sports talked to Edgar -- who fought and beat BJ 3 times -- about Penn's future in fighting.
Frankie doesn't hold back.
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