What makes a rookie an immediate starter in the NFL draft?
There are several factors actually, but none provide a singular, definitive answer.
If an NFL team doesn't have a reliable existing starter at a position, then certainly a rookie is more likely to be pressed into a starting role.
If the rookie exhibits tremendous initial skills, he is also extremely likely to crack the starting lineup.
Prior to the lockout and collective-bargaining mess, rookie contracts would also unfortunately have a bearing on which players start and which didn't. For example, you weren't going to pay Matthew Stafford or Sam Bradford $40-50 million dollars in guaranteed money and have him sit on the bench for very long.
However, since a rookie wage scale seems to be a virtual certainty with any new collective-bargaining agreement, teams should feel less pressure to start early draft picks.
Obviously, not all positions on a football team are weighted equally and it takes a lot more for a rookie quarterback (especially one not drafted early) to start immediately than it does a running back because more learning must occur before the player is ready for the NFL.
The uncertainty of the NFL labor situation further complicates the learning process for rookies and if no agreement is reached anytime in the next few months there just won't be many first-year players capable of stepping in and contributing anything significant.
Regardless, here are a list of 10 rookie prospects that I believe are the most likely to start from day one of the NFL season.