One of the postgames shows reported that Rivers could have gone back in late, but since Volek was on a roll he didn't push to get back in at QB.
To me that screams "Rivers is not franchise QB material mentally." Can you imagine Brady not fighting to stay in or get back in after an injury if he was at all able? Your QB needs to have confidence he's the best guy to win the game all the time. Rivers looked like someone who didn't want to be blamed if they lost. That lack of mental toughness is bound to show up when it matters most.