No, it's not crucial. It'd be nice, but the OL is nothing like, say, S or RB in terms of dire need. For starters, true frosh OL almost never contribute, so the key is looking ahead two years.
In two years you'll likely have:
5th Year: Harrell(?)
Seniors: Elmer, McGlinchey, Montelus(?), McGovern(?), Bivin(?)
Juniors: Nelson, Bars, Mustipher, Byrne
Soph: Hoge, Rhuland
In three years, next year's class of players will be sophomores and ready to play, so you can compensate by gaps in '15 with guys from '16. So yeah... you're looking thin at tackle for the 2016 season if McGovern and Bivin were to both be out... be even then it's not exceptionally thin or dire. You've still got 3-4 bodies before you'd get to freshman.