Once upon a time, LSU head coach Les Miles said something along the lines of "Not only do we lose a senior class every year, but we also lose a junior class." That statement is pretty powerful because it's true. Teams like LSU, Georgia, Alabama and Florida lose a ton of talent on a year-to-year basis to the NFL.
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Also, with every recruiting class, there are at least a handful of players who will transfer or get kicked off the team for various reasons.