There is this premise among some fans that when you draft a player that demonstrated a specific skill set in college, that is all you will get throughout their NFL career,
The fact is every player in the NFL is a student and the coaches are the teachers. It is a constant learning process throughout the player's career. Of course, there is always different speeds at which the player learns based on physical attributes as well as intelligence.
Having said that there are also expectations that the player will demonstrate an expected level of improvement that justifies such an investment.
Tony Romo is the perfect example. He stuck around for over three full seasons without throwing a single NFL pass by justifying his roster spot with impressive results throughout the preseasons. Romo's potential continued to be developed under Parcells and Peyton until he was ready to compete as a starter.
Both Rush and Dinucci are unfinished products. The only difference between the two of them is the way they responded to the challenge of justifying their roster spot on the team. One did and one didn't.
Still, your backup shouldn't be a developmental player. They should be either a proven veteran that is willing to continue their career as a non-starter or a young player on the fast track to potentially being a starter. The question is, does this sound like Cooper Rush? In order to answer that question, I need to see more.