Cena being gone is a blow, WWE should accept that and then start thinking about the positives. He's usually on the show for at least a twenty minute segment each week, so that's twenty more minutes screen time for someone else now. It is an opportunity to develop a promising wrestler, a chance to expose someone else to the audience. Now that may not instantly have an effect on ratings, but it will start a process of conditioning the audience to perceive someone else as a big deal. Give another wrestler Cena's spots and they'll start to get over. In the long run, ratings will follow. All the wrestlers on the roster should be aiming to grab some of Cena's screen time. His absence is an opportunity for someone to step up.
