Like a loser
I went & did some of the math for games without a starting third down back, in this case my study was the 2013 season in which the Pats lost Shane Vereen for 8 games. In games Vereen played the Patriots averaged 29.2 ppg and in the games he missed they averaged 26.3, so field goal difference.
As I like to do when I work with stats like this, I like to adjust for outliers - in this case I discarded the high & low score of each test group (a 55 point outburst vs Pit, a 6 point egg vs Cin, a 16 point yuck vs Den & a 43 point performance vs Ind) and we have the pats scoring 29.1 with Vereen and 25.0 without him.
So there's a bit of insight into the effect the loss of Lewis may have on the offense...