Woakes was never going to play and Stone can't be relied on to stay fit so I'm sure that was factored in. But it's not so much the unavailability as it is that the players coming in to replace them aren't performing. Bresnan and OHD have taken 27 wickets between them; the other bowlers put together have taken 18 (including 4 by Stone, who only played one match).
But mainly it's the batting that's letting us down. We've had 1 ton and 2 50s. Apart from Bresnan, everyone averages under 29. Bell averages 8.6 and Hain 4.4.