Previewing the next episode of his "Coach Prime" documentary with Barstool Sports and SMAC Productions on his social media page ... Sanders revealed the big toe and the second toe on his left foot had to be surgically removed several weeks ago.

Sanders showed the mangled foot in the doc's video teaser ... writing in the caption, "I endured a lot while I was in the hospital during our historic season but I am so thankful for GOD being there to help pull me through."

As we reported ... Sanders initially had surgery to fix a longtime foot injury back in the fall, but his recovery process never quite got on track.

Sanders struggled to get his foot to heal -- and he needed a wheelchair and a scooter to get around for months.

The former Dallas Cowboys cornerback said he ultimately required 8 surgeries to try to get the foot right ... and it's clear, one of those procedures included cutting off two digits from his left foot.