According to PWInsider.com, while the knee "injury" John Cena sustained at the hands of The Wyatt Family on Raw this week was part of an angle and was not legit, Cena did legitimately suffer a groin strain on Raw.
Following his bout against Randy Orton at last night's WWE Live Event in North Charleston, South Carolina, Cena got on the mic and apologized to the live crowd for not giving the performance that he wanted to give. He noted that he had indeed suffered a groin injury and was advised to stay home and rest.
According to a fan in attendance for the show last night, Cena was said to be "pretty bad off" during his match against Orton, but he toughed it out and worked the entire match since it's WrestleMania season.
Additionally, it's being said WWE wanted to pull Cena from this weekend's house shows due to the groin injury, but he refused and insisted on working the events.