I'd always be cautious about those kind of figures. I'm no statistician, but I don't think you can judge Boyd's goalscoring worth to the team based on how many goals we score when he doesn't play. For example, Boyd hasn't featured at all in only three of our last 50 league games; of those three games we scored 7 goals (4 vs. Celtic, 3 vs. Hibs, both away). However, we failed to scored against Celtic in the third of those three games. It would be easy to argue that the first two games prove we score more goals away against the likes of Celtic without Boyd, but we have so little evidence of how many goals we'd score over the course of any prolonged period without him that the data becomes a little skewed.
It wouldn't prove anything either way, but I think it would suggest there's been a big difference between this season and last season.