First, a dose of reality. Chris Johnson still has most of that remarkable breakaway speed, but he's not the guy he was when he was setting the league on fire. Never a reliable every down back, his success has always been dependent on the big play — according to Pro Football Focus data, during his 2,000-yard season he gained over 1,000 yards and scored eight touchdowns on just 30 "breakaway runs," defined as carries of 15 yards or more.
But those breakaways have declined drastically over the last couple of years and Johnson only put together 14 of them for just 271 yards. When a decline is that precipitous, it's understandable to look to the offensive line for an explanation. Except Tennessee was an above-average run blocking team in 2013, one of the best since Johnson has had since he entered the league — Johnson had no backfield contact on 80 percent of his runs, so it's not as though he had no room to operate.
One more interesting note: per PFF, he averaged 3.0 yards per carry before contact on those runs, a mark that's just above the league average, yet he only managed 5.0 yards per carry total on them, a career-low and well below the league average. Put simply, Johnson isn't doing much after first contact, even when he has a chance to get some momentum.
Given his limitations, it would seem smart to pair him with a bruising every-down guy like Ivory, and bring in the lightning after thunder has worn the defense down. But Johnson needs volume to create big plays, catching a defense's mistake every once in a while and making them pay dearly. For all his straight line speed, he doesn't show it much in tight spaces and doesn't have elite lateral quickness — this isn't going to be Andre Ellington in Arizona, or C.J. Spiller for his first couple years in Buffalo.
He's still a dangerous player, but one that comes with some risk and most likely more than a little frustration. The West Coast master Marty Mornhinweg will have to find interesting ways to get him in space, but his role as an every down back seems like a question mark at this point.