I'd say Izzy is closing but you never know. In an article this year, Walt Jocketty (GM) acknowledged that it was hard to compete in the postseason with 2 fresh rookie starters (Reyes and Wainwright). If they follow through with this, Wainwright might stay in the pen. Though, my bet is Izzy would be kept as closer and Wainwright as the set-up if this were to happen.
...Boston papers now and then suffer a sharp flurry of arithmetic on this score; indeed, for Williams to have distributed all his hits so they did nobody else any good would constitute a feat of placement unparalleled in the annals of selfishness. -Updike