You don't need a good grip on anything to stop yourself from going down unless you're completely uncoordinated. You just need to able to steady yourself on something to regain your balance. That's what Marquez did. You can see in one frame he is already leaning back on his heels and he wouldn't have been able to regain his balance without steadying himself with the ropes. I think the rules just say something like if the ropes prevent a fighter from going down, it's a knockdown. In this case, the ropes prevented Marquez from going down. I don't know where you came up with this getting a good grip requirement. Also, you don't need to be "hurt bad" to be knocked down, you just need to lose your balance. Marquez wasn't really hurt that bad, but he did lose his balance.