Given that the Sox are a higher payroll team, with Clement and possibly Schilling coming off the payroll next year, they could more readily accept Buerhle's longer-term cost than many other teams.
Presumably, if they trade for him they are at least serious about an extension. If they win the bidding for Buerhle, it means they beat other teams seeing him as a long term solution.
From Chicago's point of view, they will get a first and a first-supplementary pick for him next year. That's two top prospects that they get to choose. So they are going to expect at least two top prospects back for him.
I look forward to seeing Lester pitch with the Sox. But Buerhle is basically what we hope Lester becomes - a solid #3 pitcher, a lefty in the rotation, a dependable 200 inning guy. A Lester for Buerhle trade just gives the Sox a much more proven, dependable version of Lester. You would have to add another prospect. I assume either a guy like SS Lowrie at AA, or an outfielder like Moss or Murphy.
Then you have a rotation going forward of:
Beckett
Matsuzaka
Buerhle
Buchholz
Schilling/Wakefield/Tavarez/Bowden
Beckett, Matsuzaka, Buerhle, Buchholz, and Bowden would all be locked up long term. Schilling, Wakefield, and Tavarez provide a potential bridge.
If Buerhle goes to free agency, the Yankees can grab him.
If he is traded elsewhere, especially to an NL team, less of a concern for the Sox.