IMO, if our defense in 2015 is going to be as strong as everybody is talking it up to be (that is, elite), then we need KMAC and/or Tuttle to at least be steady contributors who improve throughout the season.
We know our defense should be able to run sideline to sideline, rush the passer, and force some turnovers. What we don't know is if we're strong enough up the middle to consistently stop the run.
O'Brien, Owen Williams, and Jordan Williams played admirably inside last season, but we didn't have the size to completely shut down the inside run game. We had to utilize a lot of stunts and move guys around. Plus, it helped that we had AJ in the middle for most of 2014. Say what you will about his lack of speed, but he was a pretty good inside run stuffer.
If our 2015 defense is gonna take the next step, then we need our big bodies, KMac and Tuttle, to be able to play effectively against run heavy teams. If KMac goes down, then that could leave us with only one 300 pounder at DT (depending on what happens with Mosley and Saulsberry, that is). I'm not sure we're ready to shut down the big boys on our schedule if we don't have a couple 300 pounders to sub in and utilize in certain situations. I could very well be wrong though. I'm no expert.
TL;DR: If KMac and Tuttle aren't able to play effectively throughout 2015, it significantly hurts our chances of having an elite defense (IMO).