Whatever ends up happening, we're fortunate in one regard with the OL: we've got three OTs who can all play at least the two OT positions. In Free's case, he can also play RG. In Bernadeau, we've got a guy who can play C, LG and RG at a tolerable level. And with Frederick, we've got a guy who can play OC and RG. That's quite a bit of position flex, overall.
With just our starting 5 OL and Mack and Costa, we can cover every injury contingency at every position with NFL-quality starters (not great ones, but these are guys who can and have started in this league). If we keep Kowalski (as I expect) and an OT (I'm guessing Bess), we can cover at least two injuries to any position with a couple of different permutations. That's really not bad depth, depending what you think of Kowalski as a third option, and what you think of Bess. Stash, say, Coughman on the PS and we can get by with 9 OLs pretty effectively.
Stash Ratliff on PUP to start the season, and you're probably buying time for a luxury player like Tanney or Johnson, at least for 6 weeks or so.
Given our injury problems with the line overall, that's actually a fairly stable allocation of resources.