Is Lamar Miller a legit No. 2 back?

Is Lamar Miller a No. 2 fantasy RB?

He's a 212-pound, 22-year-old track star who's the leading candidate to start at running back for an intriguing Miami Dolphins offense. In limited action last season, he averaged 4.9 yards per carry and put down a few highlights on tape that any observer could get excited about. He's reportedly improved in pass protection during his second NFL training camp. He's Lamar Miller, and I feel like I should love him more than I do.

Miller fits the profile of a breakout candidate. The traditional path to fantasy stardom for a running back is to be a bruising, TD-heavy, volume player (think Marshawn Lynch or Alfred Morris). But recent years have convinced the savvy fantasy owner that another path is available: the slender sprinter. Such players have dotted the NFL's history (Bob Hayes, Curtis Dickey, Michael Bennett et al), but we've seen a surge in today's game. Lately, Chris Johnson, Jamaal Charles, C.J. Spiller and Jahvid Best have all made big fantasy impacts -- Best's certainly would have been bigger without his spate of concussions -- in relatively thin bodies. In fact, despite having straight-ahead wheels that compare favorably to these other four sprinters, Miller is between 10 and 20 pounds heavier than all of them.

But measurables aren't everything. One thing Miller doesn't have that these other four players did is a lot of game tape. Miller had one year as a collegiate starter at the University of Miami and left as a redshirt sophomore. He has 51 professional totes. I've been taken to task by some Twitter followers for not acknowledging Miller's upside by ranking him well inside my top 20 RBs for 2013. But I think he carries too much risk to go that far. In my personal ranks, he's the No. 24 RB, one spot behind Reggie Bush, the man he's replacing in Miami.

You might recall that Miller was considered a potential late-first-round pick in the 2012 draft but that he fell to the fourth round. Draftniks were initially hard pressed to explain this drop, but it turned out NFL teams were nervous about Miller's injured shoulder and his limitations as a pass-blocker and short-yardage option. The shoulder is no longer a concern (with surgery more than a year ago), but what makes us certain that Miller is ready to excel on the physical side? He has two career regular-season carries on third down with less than three yards to go. According to ProFootballFocus, he's forced six missed tackles in his career. Of course, because someone hasn't done something doesn't mean he can't. But there's a reason general manager Jeff Ireland keeps relentlessly talking up third-year RB Daniel Thomas, and I think it's because Ireland knows he'll be better off if Miller has a more physical complement. The jury is way out on whether it can be Thomas, who's busted hard in two pro seasons, but that doesn't change the GM's concerns.

You might ask, so what? So what if Miller isn't ready to be a physical player in the NFL? Is Johnson physical? Is Charles? If Miller is ready to break long game-changing TDs, say, seven times this year, it won't matter. You can't teach his kind of speed, plus he seems to be a nice fit for the Dolphins' zone-blocking arrangement. Miller doesn't necessarily need to pound into the line and make something himself nor does he need to juke around defenders. Instead, he'll need to display good instincts, strong vision and decisive acceleration into the hole. I'll admit, I find the positive side of this disquisition compelling. I like Miller more in an offensive system that lets him cut once and go hard rather than weave or juke (a la Spiller). It seems like a better match for his strengths. Miller is not someone I'd characterize as elusive.

But do these positives make him a clear No. 2 fantasy RB? For me, not quite. Miller hasn't done it before. Ryan Tannehill hasn't done it before. And while Bush did have success running in Miami over the past couple of years, the offensive line has changed. Jake Long is in St. Louis and will be replaced at left tackle by shaky Jonathan Martin, and guard depth is scary with both John Jerry and Lance Louis battling injuries. In fact, if you were going to note one area of real concern on the entire Dolphins roster, it would be the team's O-line, and becoming a great zone-blocking unit requires continuity as well as skill. It's unlikely that the team's Week 1 starting five will play a single snap together in the preseason.

I absolutely acknowledge Miller's exciting upside. I can't get too ornery if fantasy owners want to shoot for the moon grabbing him as their No. 2 RB, because winning fantasy titles is often about players reaching high ceilings. But we have to acknowledge that Miller's downside is frightening. He's an unproven rusher who doesn't play a physical game on an offense that was 27th in both total yards and points scored last season. It's well within the realm of possibility that Miller could leave you holding the bag with a 600-yard, three-TD season. For that reason, I think of him more as a standard-league flex than as a No. 2 runner.