Are you sure?
week 1-6 4th quarter touches
week 2-6 4th quarter touches (initial ankle injury)
week 3-0 4th quarter touches (re-tweaked ankle)
week 4-3 4th quarter touches
So, he went from 12 touches (average of 6/game) before he hurt his ankle to 3 touches (average of 1.5/game) after.
While we can try to deduce why the usage has changed (His ankle gets worse as the game goes on, Reid doesn't trust him, Reid does trust Darrell, Reid wants to preserve McCoy, McCoy is old and can't handle 4th quarter touches, isn't the 2-minute offense back, etc), we can't logically say his usage hasn't changed since his ankle injury. It obviously has, with regards to his 4th quarter work.