Prospect eligibility
Prospects do not have to be rostered until they reach one of the following: A) 75 career MLB at bats, or B) 50 IP or 15 total appearances at pitcher (whichever comes first). Players who have not yet reached those totals FOR THEIR CAREERS must be kept on your prospect roster until you add them to your Yahoo team. Prospects who reach those limits and have not been placed on your Yahoo roster become free agents. If a player reaches their prospect limits, a manager has 7 days to put them in their lineup. Further rules at the bottom of this post about NA's.
RULES for NA players: Once you have promoted a player to your Yahoo roster, you cannot move him back to your prospect list UNLESS HE BECOMES N/A until the off season. If a player under those limits becomes N/A during the season, you have the option of moving him back to your prospect list and do not have to promote him again until he reaches the career limits. N/A over the career limits cannot be moved back to your prospect roster.