I think it's worth noting that the defense went from 16th in the league to 31st last year. While we did lose Glenn for several games and lost posey, we did replace stevens with mcree. Despite the differences I don't think anyone will disagree that the DL injuries played the biggest factor in our defense's plummet down the rankings.
I disagree with the notion that 3-4 DLinemen are somehow less important in our scheme. I think this is a myth due to the fact that they typically rack up fewer stats then a 4-3 linemen. Fewer stats does not mean less value. The front 7--really the entire 11--act as a unit and the saying 'you're only as strong as your weakest link' is very applicable to defense in the NFL.