Post.A
concussion-injury to the brain or spinal cord due to jarring from a blow, fall.

enthusiasm- absorbing or controlling possession of the mind by any interest or pursuit; lively interest.

1."last year you were in Seattle, Rick, and now you're in Cleveland, the Browns, remember?" it showing that he doesn't know or he forgot what team he was playing for.
2."like someone crushed my skull". feeling like his head is going to explode.
3."Monday. The game was yesterday. Do you recall any of it?" He doesn't remember any thing that happened to him during the game.

One theme of the book is that you have to keep doing what you love and never give up because if that's all you know then go with it and work at it.