A lot of fans were disappointed when John Cena didn't make a surprise appearance in the Royal Rumble match and, as you may recall, the original plan was for him to be written out with an injury after a backstage attack from Lars Sullivan. That didn't happen when the latter went the NXT Superstar and Cena was instead taken off television with the excuse that Drew McIntyre re-injured his ankle.
Now, Brad Shepard of the "Oh You Didn’t Know" podcast has shed some light on that. "So there was the story of John Cena being advertised for the Royal Rumble and then right before the Royal Rumble they announced that he was injured and would not be in the Royal Rumble match."
"There was a lot of speculation on was this a work, was this a shoot? You know was it a work to sell tickets, etc I spoke to a source in WWE and the confirmed that Cena’s injury was not only never legit, he was never legitimately injured, but it is dead on that WWE false advertised Cena knowing he would never show up to the Rumble just to move tickets. That is confirmed."
During the PPV, it was revealed that Braun Strowman took Cena's place in the match and while that may have satisfied some fans, it was a rather dishonest move by WWE to trick fans in this way.