According to F4WOnline.com, the word going around WWE is that John Cena is suffering from a back injury.
Reports indicate that Cena is suffering from a few bulging discs, which has created some discomfort. As nothing more about the reported injury, such as which discs are affected, has been released, the severity of the injury is hard to assess.
Nonetheless, it does appear that the back issue was responsible for WWE dropping plans to hold a John Cena vs. Sheamus match at the Royal Rumble.