Unless ... he's actually not a not-zombie. According to multiple fans on multiple forums and subreddits, Rick has the virus that turns perfectly good corpses into walkers, and it totally turned him into one. He actually died days into his water-free coma, because George Romero invading your universe doesn't mean basic science suddenly ceases to be.
Since those with the virus become zombified as soon as they die (bite or no bite), it's feasible that Rick's coma significantly weakened the virus -- just not enough to cancel out his resurrection. So he awoke undead and (presumably) immortal, but with his mind and humanity still intact.
He forgot how to hold a gun, but that's probably not important.