A number of fans, myself included, have wondered why Derrick's game so dramatically has fallen off this season, until very recently. Well, Pop disclosed in a recent pre-game press conference that Derrick "has been hurt most of the year" and has been operating at 1/2 to 3/4 speed. He's slowly building confidence in his (surgically repaired) foot. This could also be a reason for his uncharacteristic lack of lift that I've noticed in his shots this season and must be affecting his rhythm and poor conversion rate (and confidence)..
Here's the link to the interview, starting just after the 5:30 mark: Popovitch pre-game interview