'Big Hungry' is still in a very precarious position. He may have won fans over with his squash victories against various disposable lower-card acts but he has yet to be tried out against a popular act like Punk. Although 'The Second City Saint' is the company's lead heel he still receives cheers from many fans, which could result in the audience turning on Ryback.
The fans only need to boo WWE’s lord of the jungle on one televised show for it to become the normal state of affairs. While 'The Human Wrecking Ball' could always be turned into a bad guy it would rob WWE of their next feature babyface. Ryback has done well to become so popular, it's a shame to risk that by pulling the trigger too early on his first headline feud.
Having Cena endorse Ryback seems like a stupid idea too. Ryback's best shot at becoming WWE's number two babyface, and ultimately the chief good guy, is for him to be presented as an alternative to Cena. Cena's backing could turn some people off of 'back, whereas if Cena were to openly oppose or disapprove of him then Ryback would gain the support of Cena's legion of detractors.
CM Punk will need to avoid this situation at Hell in a Cell
Cena could voice concern over someone as reckless as Ryback being used in such a high profile spot (dripping with irony, but never mind). He could have been booked to beg and plead with Vince to put him in a match with Punk even though he's injured, only for Vince to imply it’s the beginning of another man’s time and go with Ryback. Cena being desperate for a title bout wouldn't have been heelish or out of character: the championship has been well established as the most important thing in the world to the Cena character and he should, as the face, be willing to work hurt to get it. Ryback and Cena wouldn't have to come to blows, but not all babyfaces have to get along.
Perhaps I'm being unfair or short-sighted (I don't want to use the word "wrong"). Perhaps 'Big Hungry' will get a booming response at HIAC and work a short, intense bout with the champ that sees him the win the title and continue his ascension to the number two babyface spot. Perhaps he will suffer his first defeat after some shenanigans from Punk, protecting him in the eyes of fans. Perhaps the two men won't clash inside the Cell structure and Punk will go for a cheap DQ loss, which would also protect Ryback and get Punk booed.
I want Ryback to become a success for WWE. He's a different sort of act to the current babyface crop of Cena, Orton, Sheamus and Mysterio. His matches may not be all that exciting but neither are Cena's. Ryback would at least be a new face, creating fresh matches and scenarios. If his outings are kept short and he's not overused as Cena has been over the last seven years then I think Ryback could be the new babyface star the company's been needing for quite some time.