Dave Meltzer discussed John Cena on Wrestling Observer Radio this morning and noted that Cena isn't expected to be at the Royal Rumble since he begins shooting for a movie that day and that he was actually never scheduled for the event despite WWE advertising him for it. Meltzer added that the "impression" he got was that Lars Sullivan was supposed to injure Cena but because that didn't happen, they made up a phantom Cena injury to write him out of the PPV.
For anyone who has been following me on Twitter (@RajahNews), I actually have wrote about this more in depth and can pretty much confirm Meltzer's report. The plan for Lars Sullivan was for him to be in Memphis last week (1/14) to tape a "non-arena" segment with Cena, where Lars would basically beat him up. The segment would then have aired last night on Raw (1/21) and served as Sullivan's official main roster debut.
Because of Sullivan's continued absence, Vince McMahon grew upset last weekend and nixed the entire segment sometime before Raw - whether he knew Sullivan was going to remain away or whether he didn't care I don't know - but he was mad and scrapped it. At the time, I did not know that Sullivan's segment with Cena was going to be worked into an injury angle where Cena would be written out of the Rumble - I had figured it was just to build towards WrestleMania. I should also add that there are two months and two PPVs between Royal Rumble and WrestleMania and so there's still a small chance Lars/Cena interaction could happen in February or March, but this is all dependent on Sullivan returning to WWE and ultimately, if Vince still wants to invest in him to that degree.
Anyway with Lars missing in action, WWE still had to get Cena out of Royal Rumble and according to Meltzer, that's why and when they announced the injury yesterday.