Get a concussion and try to count by 8s. It's realistic. I've had 7, and there's no way I could remember an NFL playbook when affected by one.
You gotta think... 50k+ people screaming, the size of NFL playbooks... you take a good crack to the noggin, if you're playing, the coach as probably already told you, "No Audibles".
Given time, yeah, he'd remember 'em, but he's trying to remember the snap count and play call already... and it's a QB, so he's trying to remember the play call for 11 players. There's only so much a concussed brain can handle, especially in a limited amount of time. Perhaps it would be more realistic to have him suddenly go silent and think real hard until you got a delay of game?