One of a defenses' primary assets is experience. I've been over this before, but I recall people lauding Stanford and LSU's system for example and both got absolutely demolished last year by the same HUNH teams they contained the previous year. What happened? It sure wasn't a change in coaching and on LSU's part it certainly wasn't a lack of talent.
It's experience! Alabama, every single year loses guys in their secondary that leave early and they get replaced by inexperienced players. Remember, players can stay on the team for 5 years really, Alabama loses a lot after 3 or 4 years and yet some fans, for what ever reason, can't comprehend why a sophomore or freshman doesn't play like a senior or junior, or why an inexperienced player doesn't play like an experienced one. Coaching players takes time, experience takes time, for every instant impact freshman, there's a Javier Arenas who went from not playing and wanting to quit, to blowing coverage against LSU, to becoming an NFL Draft pick.
Alabama started a true freshman one game at CB last year. Now, I'm trying to come up with a shut down corner as a true freshman, but it's hard to do. There's a reason that happened though. Alabama has had a secondary guy leave early for the 2012, 2013, 2014, and 2015 drafts. Prior to those seasons though, I can't recall the last time that happened. We're talking about in some cases guys who were only on campus for three years, some like Landon who only actually started one full season. It's inevitable that a secondary facing inexperience and the loss of quality players have adversity.