In the three games Jax has played since Dareus has joined them, they have allowed 166 rushing yards (53.3 per game) on 65 carries (2.55 per carry). Prior to his arrival, their rush defense was at the bottom of the league.
The Bills' D had one solid run defense game against Oakland, who just happened to be missing their most talented RB that game. Since then, they have not been decent, to say the least. It's hilarious to think that giving up 146 yards and 2 rushing TDs on the ground represents a massive improvement over the previous two weeks.