There is a natural learning curve inherent in making the jump from the college to the professional ranks for NFL players. The quality of competition is superior by exponential levels, with the pool of talent whittled down to just the top few percent of all college players in the nation.
They are bigger, faster, stronger, smarter, more experienced, and own just about every edge conceivable over the average rookie. It can be something of a culture shock for rookies to adjust to the intensity of an NFL game. While some have success early on, of course, it never comes easy, regardless of what Leonard Fournette might have said.