While Mr. Papelbon exploded onto the scene this past week, I think perhaps it is early to jump on his bandwagon. I can think of quite a few young studs that came out like gangbusters as Papelbon has, only to fall on their face. Also your mentioning of him possibly going to starter is a valid point.
There is a young man right now on the Phillies named Ryan Madson who was a stellar reliever and then Philly lost their closer to free agency. Shockingly they brought in an old closer, Tom Gordon, instead of giving the job to Madson, opting instead to put him in the rotation. It is clearly too early to gauge how successful this move will be, but it hurts the mans value nonetheless because of the unknown. I feel the same thing may happen to young Jonathan if he too is moved into a starting role as Boston seems to have planned.
While 40 saves of 127 is a much greater percentage than the 35 HR out of 286, their relative value is not even close to correctly represented by those stats. I wont even bother to try and properly compare the 2 because I feel this debate is not meant to have us compare the values of Shelton and Papelbon.
While Papelbon is a good example of finding a closer off the wire, he is not the ONLY one. Throughout the course of the season, past stats lead me to believe at least 6 more men will emerge from the unknown to become closers and succeed at doing so. There is a reason people wait so long to draft closers.....they really only truly lend themselves to one category, and their jobs are never a certainty. Papelbon has about as little security in his closer position at this point as anyone. While I am not a Keith Foulke supporter, I would be shocked if they pay him the big bucks to set up a rookie. Moving around from closer to reliever to starter to closer and so on may be very taxing on young Jonathan and in the end I think it will get the better of him.
The fact that Papelbon is in Boston and Shelton is in Detroit is really going to swing the star quality of each man as Papelbon will get tons of media attention and Shelton almost none, despite the fact that Shelton will have a much greater chance at success since he already has a full major league season of fine production under his belt, while Papelbon has a WEEK. To suggest that Papelbon will save 40 and become a star based on one week of play is insane, and personally I am not ready to commit to his excellence at this point.