Jon Lester isn't unreplaceable. At all. It's just that to replace him will either (a) cost similar amounts of money on similarly aged pitchers (Shields, Hamels, etc.), and if that's the case, what's the point in letting Lester go? Or (b) take a lot of time, as Owens (who *might* get to Lester's level) or another one of their bright pitching prospects matures and gets to that point. Do we want to wait long enough for (b) to occur?
A 2015 rotation of Lackey, Masterson, Buchholz (assuming he rebounds), RDLR, and Workman isn't the best rotation in the league, but it's solid for sure.