WWE.com noted, "Among the many surprises of tonight’s Raw was an unexpected look at an injury the WWE Universe hadn’t even known WWE Champion John Cena sustained:"
Despite the injury, WWE.com claims Cena's title defense at SummerSlam, as of right now, is not in jeopardy. "For now, it does not appear that Cena’s title defense is in any danger, nor is his eligibility for SummerSlam. He can and has fought back through worse. As for whether he’s truly at 100% when he puts his title on the line? That, at least, is no longer a given."
Check out the complete article at WWE.com.
For what it's worth, Cena's elbow has been in rough shape for awhile now. When he had surgery last year, doctors were said to have been "shocked" that he was still able to perform and lift weights. We're told he tweaked the elbow on the most recent overseas tour. That is why it was so visible on RAW.