Note: The below is pure statistical garbage because numbers don't mean anything and Bill Gates can't tell you which RB has the best vision and durability and therefore, which RB will turn into the next stud.
I compiled a dataset of rookie RBs in NFL history that were "productive" in their rookie seasons and then ran a regression based on draft status, rookie production and yards per carry as the input variables, with NFL career value as my output. Huh? Who cares.
Basically, over 0 is a pretty good year, 41 is what Ryan Grant was last year, 304 is what Matt Forte was last year, 699 is what Michael Turner was last year and 1300+ is what LT was in his best season.
For NFL draft value, I used the draft value chart.
The best fit regression formula was:
1245 + 0.86*draft value + 1.88*VALUE -23.9*YPC
With Matt Forte's rookie inputs of 460 for draft value, 304 for VALUE and 3.92 for YPC, we get a career value grade of 2118. What does *that* mean? That's exactly what Deuce McAllister's career value score is, and puts him in a tier with Terry Allen, Robert Smith, Floyd Little, Rodney Hampton, George Rogers, Warrick Dunn, James Brooks, James Wilder, Joe Morris, Garrison Hearst and Thomas Jones. I have no idea if that's "good" or "bad" based on what we know so far about Forte.
What I find interesting is the interaction among the variables. The #1 overall pick is worth 3000 points, so that's a draft value of 2580 right off the bat. So even after one year, being the a top five pick is worth a lot. Why? Because Tony Dorsett, Jamal Lewis, Marshall Faulk and LT were not ZOMG world beaters their rookie year but became future studs. They weren't bad, but they obviously got much, much better. Conversely, guys like Don Woods, Mike Anderson and Reuben Mayes were awesome as rookies, but were low draft picks that flamed out. So draft status is still pretty important after just one season.
That said, the other interesting thing is the YPC variable is negative. That doesn't mean YPC is meaningless, it just means that when you are using draft value and my RB value ranking as your inputs, bringing in YPC to the table doesn't add much. My RB value system ignores YPC but focuses on rushing yards, TDs, fumbles and receiving yards.
For example, LT and Curtis Martin had lackluster YPC as rookies. So did, to a lesser extent, Marshall Faulk, Eddie George, Billy Simms and Edgerrin James. Ickey Woods, Don Woods, Mike Anderson and Reggie Brooks had great YPC averages as rookies but didn't do much for their careers.
What's this all getting at? A RB who was a very high draft pick and had 1800 total yards and just a 3.6 YPC average after his rookie season would look like a future HOFer to me. Of the 12 or so most productive rookie RBs in history, the only two that flamed out were sixth round picks -- Anderson and Woods -- and they were the only picks after the third round of those twelve. Looking deeper, Olandis Gary and Mike Thomas would fit that as well, although so would TD.
It's impossible to find anyone that matches Forte's profile of 2nd round pick, great production, low YPC as a rookie. Eric Rhett, Curtis Martin, Anthony Thomas and Matt Snell come the closest, and they aren't all that close.
Of the most productive rookie RBs with a low YPC average, ignoring draft status, you get John Stephens, Curtis Martin, Marshall Faulk, Eddie George, Billy Sims, Olandis Gary and Edgerrin James. There's a lot of talent in that group, but all but two were first round picks.
Anyway, I've probably said a bunch of nothing here, and I doubt you're still reading, but my main takeaway is considering Forte's strong production (i.e., 3rd in the league in yards from scrimmage), his low YPC is meaningless and shouldn't turn you away; however, a red flag may be that low YPC average in conjunction with his non-high draft status. Only a few second round RBs have been very productive -- Portis, Ricky Watters, Ickey Woods, Corey Dillon -- and they all had huge YPC averages. A-Train would probably be the closest fit there, but his YPC average was better and his production was worse.