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Mark Sanchez is grieving the loss of his former teammate Nick Mangold ... sharing a heartfelt post to social media amid his own legal woes.

The former Jets quarterback posted to X hours after news broke that his former center passed away from kidney disease ... writing the world lost a very good football player -- and an even better man.

Sanchez believes his pal will end up in the NFL's Hall of Fame eventually ... but, adds his legacy will be so much larger than that -- 'cause he made a lasting impact on so many others.

Sanchez calls him, " my big brother who introduced me to the NFL. He showed me the ropes. He was my friend and confidant. He was a role model and a great father to his children and husband to his wife." Mark ends the statement by simply adding he will be m

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