If Knowles plays - big if - then he'll likely have some protection/strapping round his thumb/webbing. Might not be a good idea to have him as DH for an extended period of the game.
I suppose Welsby could go to half back and Fages to hooker if necessary.
I'm personally a bit jittery about Percival. On form, he's such a superb player that you need to have him playing, but there's still some questions about his hamstring IMO. So a contingency option is necessary on the bench.