Portis 3-4 years ago would still have been able to get yardage behind this line. Perhaps not on an every down basis, but breaking the 10-15 yard runs when the channel was open. That's what Betts did yesterday.
Right now, Portis is just not the same back he was in 2005 or 2007. He is older, slower, heavier and has some serious injury problems beyond the concussion. The ankle is probably going to require surgery after the season.
I'll bet that Portis is no better than a 4.6 or 4.7 rusher now in terms of straight line speed with the ankle and he has no quickness to get beyond the first level of the defense.