AJ Lee returns one or two weeks after her concussion. Cena returns 4 months early. Either WWE doctors misevaluated their injuries, they are being rushed back or in Cena's case they lied about how long he would be out just to make him look super powerful to overcome an injury.
At the same time both Cena and AJ are the most popular in their respective divisions. It makes sense they'd want to rush them back,
Yes and it's their own fault for not developing better characters. Everyone has been in the main event picture once then pulled way back down. Miz was doing well, Kofi was too, Fandango then all of a sudden they are gone, loosing matches, out of main event. Because of that they have to rush their biggest names back risking more losses.