Cena's injury wasn't the only reason why ratings drop. A lot of people like to point to one wrestler or one storyline as to the reason why ratings drop. It's not. It's a lot of different things that contribute to a ratings drop.
I believe ratings are low right now because of a lackluster No Mercy. That show was absolutely bland. Felt like a regular episode of Raw.
It'll go up a bit this week because of curious fans wanting to see what Michaels is going to be involved in, though.