Athileptic

An athileptic is a person that believes they possess great athletic skills, but their in-game performance more closely resembles an epileptic seizure.

Can be used as a noun or an adjective.

Tyrone: I thought you said Mike could play?

Petey: He said he could.

Tyrone: Looks athileptic to me.

Petey: I won't pick him up next week.