an athlete playing his or her first season as a member of a professional sports team

The rookie replaced the injured regular at first base.

a raw recruit, as in the army or on a police force.

a novice; tyro.

noun

amateur , apprentice , beginner , colt * , cub * , fledgling , freshman/woman , greenhorn , neophyte , newcomer , new kid on the block * , tenderfoot * , trainee , abecedarian , freshman , initiate , novice , novitiate , tenderfoot , tyro , buck private