I more or less c/p'd this from the comments of MCal's most recent post.  Read it if you haven't, because it's great.  He's great. 

Anyway, I was just idly thinking about how Troy Woolfolk's injury in an instant dashed our Motor City Bowl dreams, worrying that 3 wins would be more humiliating than 4.  But unless my method sucks, it looks like most college football starters simply can't do that much* to affect their team's chances.

Let's say on average the worst team in college football wins 1 game.  Call that replacement level.  Every school in college football has at least the talent level this worst team does.  That means an average team has 5 wins above replacement.  Football Outsiders breaks wins down using a 40/40/20 rule. That is, 40% of wins are attributable to defense, 40 to offense, 20 to special teams.  So an average defense will be worth 2 WAR (40% times 5 WAR).  If each player is about as important as the other (probably true on defense), then an average defense will feature a unit of ~.2 WAR players (2/11, rounded).  Average defensive players are worth just 1/5 of a win above the talent of the worst college football team.

To round out that scale, we should still try to get some idea of how good the best players are.  The approximate range for defense yards per game allowed is about 200 ypg to 500, with 350 about average.  The difference between best and worst is twice as much as the difference between average and worst.  As long as we assume that yards convert linearly to wins, it looks like the best defense (4 wins better than the worst team) would have on average ~.4 WAR players.  Even the very best defenders are going to max out in all likelihood around 1 win above the worst players.

So Woolfolk in himself won't mean the end of the season...unless we can't supply replacement level players.  And given our depth, maybe that could be problematic?  On the other hand, as MCal pointed out, are we really going to be worse than various other terrible outfits around the country?  Northwestern and Indiana are always dealing with these kinds of problems.  Achieving replacement level should not be a significant hurdle.

On the other hand, let's look at what we can expect from the defense this year given what we now know.  The offense last year was about average.  Special teams were above.  Exactly how bad was the defense?  Wholly average teams get 2 WAR from defense, 2 from offense and 1 from special teams.  So let's say we got a full 2 WAR from offense and 1.25 ST wins.  If we were a true talent 5 win team (i.e. Michigan won 5 games because they weren't unduly lucky either way), that means 1.75 wins from the defense.  I think we can probably assume BG was worth .75 wins in himself.  Martin, RVB, Roh, Brown, Woolfolk, Warren were varying degrees of not horrifying.  Kovacs, Floyd, Ezeh and Mouton were all near replacement level.  In fact, let's run with that.  1 win split among the actual contributors (Martin et al.) still means slightly below average talent in that group.  If BG's dominance was not so great, they'd come out better of course.  This isn't the most robust analysis ever, but I think most observers would paint BG's season as seriously that good.

And if there's one thing we're sure of, it's that losing one dynamic player isn't that big a deal.  Depth can absolutely make up for a lack of stars at the top end if all you're trying to achieve is competence.  I happen to think the offense will be well above average and ST will be fine.  The question, I think, comes down to how dominant the DL can be and, after that, finding guys who can do a couple things who maybe have some flaws in their game.  Moundrous may have trouble walling off a slot receiver on a vertical, but if he can stuff the run he'll have value over Ezeh last season.  And then maybe Demens can do the pass drops/blitzes on passing downs.  Piece by piece, use everybody.  That's the way we'll have to do it.  And just maybe we'll see a bowl game this year. 

*Exceptions granted for quarterbacks and, in case Brandon Graham is reading, Brandon Graham.