After the 2006 Super Bowl, when Shaun Alexander's contract status was still in limbo (but not with the baby souls, the pope took care of that) I was among many who championed not resigning Alexander, but instead giving Maurice Morris a shot. A lot of this might be explained with simple DVOA:
Before Alexander's historic 2005, Morris outproduced him per carry for every season since Morris was drafted. It seemed logical to keep a very good running back, one, seemingly, better for his career than Alexander, but Morris' production always came as a backup. Then came last season...
With his first chance at regular carries in his career, Morris' measured performance mirrored the entire rushing offenses' decline and still topped Five Fumbles Alexander, but he looked worse. Running back is perhaps the only position in football where you can see the entirety of their contribution.* You can see them block, in the case of Seattle you can see them get bitchslapped out of the way by a blitzing linebacker, but most importantly you can see them run. You can see how quickly they hit the hole, how instinctually they see cut-back lanes, their second gear, and their ability to square their shoulders and hit the defender low and hard.**
Morris is a single cut rusher. Sort of like Tatum Bell without the elite second gear, but better stamina. His best trait is his ability to hit the hole cleanly and with authority. An ideal Morris rush involves a quick dash to the line of scrimmage, a single cut towards the most open part of the field before being corralled by defenders 10-15 yards down the line. It's a style as underwhelming as it is undemanding on the offensive line. Morris rarely cuts back, he doesn't need great pulling guards and he rarely busts a big one, but he does get productive yards and matched with the right offensive line he's valuable. Thus the DVOA.
Alexander is a better big play back. Excluding 2006 where he was presumably injured all season, Alexander averages a run of 20 or more yards for every 30.12 carries. Morris, 47.43. I'll take steady production over boom and bust any day, but a good back should be able to do enough of both. The best NFL rushers are able to hit the slightest seem with authority and turn holes into big gains. Morris succeeds at the former but fails at the later and therefore will never be much more than a spell/change of pace back. He is also a poor blocker and when thrust into starting duties last season transformed from a mere cipher in the passing game to a true negative. Just a terrible receiver.
Much has been made recently of the importance of a running back versus the importance of an offensive line. Naturally, five guys will have a greater impact on the rush offense than one, but I think, proportionately, running back is still the single most important position for a good rushing attack. What we can know with some certainty is that once a rusher is ten or more yards down the field, he's mostly on his own. I expect Morris' production to rebound along with the rest of the rushing offense thanks to Rob Sims and Chris Spencer replacing Chris Spencer and Robbie Tobeck as the starting left guard and center, respectively. I just don't expect Morris to ever be a feature back.
*Counterwise, wide receivers and defensive backs do the most important part of their job in anonymity: get open/maintain coverage. When you do see that awesome game of cat and mouse it's chopped into instant replay and so isolated that it's impossible to put into context. That's one of the reasons that secondary play is still the hardest thing in football to accurately judge.
**That's what she said.