We all know by now that with Fox's scheme we are gonna be applying a 2 back system. Moreno will likely be half of that, and the other we may acquire in free agency.
One of the reasons Chris Johnson had such a bad year was that defenses knew, okay he is fast fast fast. So they play fast.
With Moreno we have a quick, elusive runner with excellent vision. He catches out of the backfield extremely well. Can break arm tackles but is not going to run over a lot of people. He is quick through the hole but lacks break away speed. I don't feel like he runs with the same powerful center of gravity that carries momentum forward.
One important thing about defense is the mentality pre-snap. For defensive players going against Knowshon, they have a different mentality than if they were against Peyton Hillis. For defensive players going against a runner like Moreno, they need to hurry to the ball and get him without getting juked. Moreno is a bit undersized so you'll have guys licking their chops thinking they can pack a wallop on this guy.
-Front 7 players understand that Moreno is a shifty runner so they already think in their heads "okay I have to play light on my feet, keep my eyes on his hips so I don't get juked out."
-DB's are thinking the same thing, except " holy crap this guy is pretty small I can impress a few people if I deck him". So they are coming for him hard and they don't fear him as much because chances are they are faster than him as well.
When going against bigger backs like Peyton Hilis, its much different. If you played defense and had to tackle a bigger guy you know what I mean. You have to gather you balls a little bit pre-snap. "Okay this is gonna hurt but I got to tackle this guy". You have to come hard and fast on him so he doesn't get momentum.
The DB's are thinking "oh crap, go for his legs"
So in summary, defenses really will get messed up if we find a complement to Moreno. One with more power that is. Having to prepare for Moreno and a big tough runner is two different things. It messes you up. It has a lot to do with your Central Nervous System, I don't want to go into great detail, but basically the moments before a big car accident you can "brace" yourself. The signal sent to your body and muscles is different than for example if your asked to chase down that quick little Japanese neighbor who you found masturbating in your garden again.
If this is hard to understand sorry I did this pretty quick, but I'll try to clarify in the comments.