well i just watched Prince Charles Williams vs Bobby Czyz. Williams looked done after being knocked down at the end of round 2 and then in round 3, but he ended up dominating the fight behind the jab and the doctor stopped the fight for Czyz in the corner at the end of the 9th because his right eye was shut.
Many of the examples given so far, do not seem to be fighters playing possum. Williams was undoubtedly seriously hurt against Czyz ditto Castro against Jackson or Moore against Durelle. Maybe I am misunderstanding the question, but I think of 'playing possum', as something being faked, in this case a fighter blagging he is OK, when in reality he is seriously hurt.