Pitching is also lacking in depth. The reliever will get almost as many wins and will most likely have a smaller whip and era. Try to find the middle reliever who is coming off a sub-par year but had set his career high in innings pitched two years ago, for he should now be balanced. Most rookies do not have an immediate impact in the league, only the Rookie of the Year, and maybe the runner up puts up useful numbers. The players with real potential usually get a lot of hype and can cost as much, or more than, a proven position player.
Most young players take three years to reach their full potential.
Most first year players run out of gas down the stretch, when you need them most. This is especially true for pitchers. No rookie relief pitcher has ever won the closer's job outright. If a team is winning, the rookie will be send down to work out his problems. It a team is going nowhere, he will be kept in the majors and suffer his growing pains, on your roster.