Not doubting his chance of winning the fight ended with the injury, and that would've explained the desperation, but I can't understand how an achilles injury can make a man go from sitting on his stool having a chat at the end of round 5 to being completely gassed at the end of round 6.
Gassed? he didn't look tired, he looked like he could barely move his leg.
I mean look up what the achilles tendon does, or any tendon. It connects the muscle to the bone, of it's snapped then you literally lose the ability to use your calf muscle to pivot the foot backwards. You can't get on the ball of your feet because you can't lift your heel up. It's just gone, the calf muscle would no longer be connected to the foot.
It's not tiredness, if the achilles has gone you physically lose 50%(the more important 50%) of motion, it's not hard, it literally does not work. Think about your walking motion, as your right foot is forward you want your left to role onto the ball of the foot then push off, that motion is physically impossible. That's why he was lifting his entire leg at the knee and slapping his foot down ahead of him, it's all that was possible.
He did amazingly well, he might have also been in really serious pain, I mean something clearly went, you have two real situations, the tendon is ripped badly or ruptured, whatever, then every step you take might be ripping it more and that would hurt like hell. If it went completely, then that is still something torn inside of you causing pain and every time you step down on it, it will hurt a fair amount.
Not a fan of frankly any of these fighters, but trying to play on with one leg not working, a severely compromised base with which to punch from and in great pain was a pretty incredibly show of bravery. Though he was helped massively knowing Bellew was no genuine threat to himself in terms of causing him a serious and dangerous knock out.