I don't understand the surprise here.
If you've listened to Rivera's comments on training cam so far he's been real high on Munnerlyn a few times (Steve Smith of the defense etc)
Munnerlyn and LaFell both.
'Course I haven't seen any of the action from cam so it surprises me to read "as bad as he's been in camp" in the OP.
As far as the coach is concerned he's almost certainly locked in the 2nd corner spot:
"I think Captain Munnerlyn has a foothold on (cornerback opposite Chris Gamble) but they’ll still battling it out for the third position."
Ummmm no, just because Rivera praised him doesn't mean he doesn't suck, they guy got owned by fuging Darvin Adams, I've never even heard anyone say Munnerlyn has even been good on ONE play.