not realistic to expect a first-time starter to produce at the same level. It does happen obviously, but it's the exception, not the rule, and Boise isn't signing top 25 recruiting classes. Look at the drop off in UCF from two years ago to last year. They kept almost their entire team besides their QB and RB and went from a BCS winning team to out of the top 25.
I don't think Boise will slip out of the top 25, but losing their QB and star RB will lead to at least some regression early in the season.