Thanks to their historic comeback in Thursday night's Game 5, the Red Sox are still alive in the ALCS. However, despite the fact that they've staged two similar October comebacks in the last four years, they still have a lot of work to do, and their starting pitchers will be a big part of that. The two pitchers that'll go in the final two games of the series aren't in question- it will be Jon Lester and Josh Beckett. But, because of the extra off day after Game 4, Boston has the option of switching up the order they pitch in, having Lester go in Game 6, and Beckett in Game 7.
They've chosen not to do this, and Beckett will be on the hill on Saturday night. The most popular argument for going with Lester will probably be that the Red Sox have the "momentum", and if they win Game 6 they'll advance to the World Series, since the Rays will be so devastated, and Game 7 will be a formality. I don't really buy into that--they need to win both games regardless, and "momentum" in baseball is really just a myth--but I don't agree with Boston's decision either. Lester is obviously the better pitcher at this point, but the most important thing is that he is more likely to go deep into the game--he pitched seven innings in both of his ALDS starts, and even when he struggled against the Rays in Game 3, he made it into the sixth.
Beckett, on the other hand, has pitched a total of just 9.1 innings in two playoff starts. If you save him for Game 7, Tim Wakefield will be on full rest as well, so you'll have a potential long reliever if Beckett gets knocked out early. Since, win or lose, they'd have at least two days off after the deciding game, they'll also have the ability to make just about everyone in the bullpen available on Sunday night. It's unclear why the Red Sox would choose to have these advantages with Lester pitching, rather than Beckett--it really seems like a poor allocation of resources--but the organization does have a pretty good history with these decisions, so we'll have to wait and see what they have in mind.