The moment, to me, Cena "got it" was at One Night Stand where he and the crowd pre-match threw his shirt in and out of the ring for 10 minutes before the match with RVD started. From there, Cena just rolled with the punches and has been better off for it.
At this point..."Let's Go Cena...Cena Sucks" is just his chant. Fans at this point just rib him. And, Cena himself just eats it up.
There are times in some matches where Cena just rolls through despite being shit on...but he never gets rattled or visibly shaken by the crowd shitting on something he is doing. Guys like Orton, Batista, Sheamus, Big Show, Kofi, Miz...they have all suffered badly from not being able to adjust or not being able to control the crowd. Not everyone needs to be Hogan or Roberts, but damn, have a little improvisation.