I heard from a reliable source yesterday that John Cena's hips are bothering him and he was originally expected to take time off after losing to The Rock at Wrestlemania XXVIII. CM Punk was slated to carry the load on Raw.
When WWE was able to finalize a deal to bring back Brock Lesnar, Cena volunteered to stick around longer to do a program with a returning Lesnar. My source said it's possible that Brock "injures" Cena at Extreme Rules on Sunday so Cena can take time off as he is really hurting right now.
With that being said I cannot confirm any booking plans for Cena vs. Lesnar at Extreme Rules and just know Cena is working more banged up than usual.