That's probably true, but they will need a big to give them 10-20 minutes until Williams comes back. Who starts doesn't matter much. That's the job that they are competing for and its a big step for whoever gets it.
Overall you're correct that they need a part-time big body to fill some space until Rob comes back, and that they can really limit what they ask of that person in order to get away with it. I think Kornet has a bigger leg up on that spot than you're presenting though, what with it being his third season in Boston and both Vonleh and Kabengele being out of the league last season.
I really wish Theis would get moved by Indiana to someplace that doesn't need him so Stevens could just go and get him yet again. Danny would really be plug and play for however long Rob needed to recover.