Starting a guy on Day 1 doesn't automatically mean "he gets better, sooner".
Often it means "He looks terrible, develops bad habits that become even harder to remove, loses confidence in himself and loses the confidence of his teammates."
Many times, not starting them right away, is the faster and more reliable way to getting them to where you want them to be.
it all depends on the individual player
Troy Aikman and Peyton Manning were starters as rookies, they threw lots of interceptions, their teams didn't win much, but both said later the experience was good for them ... they were mentally strong enough to absorb the abuse and actually learn from the experience
then you have JeMarcus Russell and Matt Leinert ... started as rookies, they threw lots of interceptions, their teams didn't win much, but neither were mentally strong enough to absorb the abuse and actually learn from the experience
so it isn't just physical ability that defines whether a newly drafted player will succeed or fail ... there is a very big mental part to it that plays a much bigger role than many understand or accept ... that is why Cad McClown was such a washout as a Bear because mentally he was on par with dryer lint