If the Red Sox fail to sign Jon Lester to a contract extension, they could seek to replace him at the front of their rotation with a trade according to Scott Lauber:
Source: #RedSox are keeping options open to acquire frontline pitcher in case can't sign Lester, but nothing doing on Hamels front right now— Scott Lauber (@ScottLauber) July 18, 2014
In other words, according to source, trading for someone like Hamels is more of a concept than a reality for #RedSox right now— Scott Lauber (@ScottLauber) July 18, 2014
Lauber locks right in on Hamels, but let's take this on the more conceptual basis he suggests in his second tweet. The Red Sox don't know if they can sign Lester to a contract they're comfortable with in terms of years and dollars. They do seem to recognize, however, that they at least need someone akin to Lester to lead the rotation--we'll avoid such loaded terms as "ace" for the moment. Unfortunately, that sort of pitcher doesn't tend to agree to the types of deals the Red Sox are looking for.
The idea, then, is to go out and find a pitcher who is partway through one of those large contracts, leaving behind a more manageable commitment that's more in line with the team's philosophy. To return to Hamels just for an example, he's got four years and $90 million (plus a team option) left on what was originally a seven-year contract.
The negatives of such a strategy aren't entirely insignificant. The Red Sox would have to trade for their replacement, which would require a serious package of prospects given the level of talent they're looking to acquire. They would be able to offset some of that by either trading Lester or, in a worst-case scenario, letting him walk and bringing in a draft pick, but they can still expect to take a hit to their farm system.
There's also the risk that comes with any outside acquisition. While there's always the possibility that any given player will fall to pieces in any given year, the Red Sox know that Jon Lester can pitch in Fenway Park against the A.L. East. That's not necessarily going to be the case for any free agent or trade target, and once you start throwing in the contract stipulations the field can start to look awfully narrow.
It certainly makes sense for the Red Sox to keep an eye on this avenue, particularly with the trade deadline so close. For all that public opinion seems to be that the Sox must re-sign Lester, these last few months on his contract represent quite the trade chip, and if the team can't or won't sign him before July 31st, they're almost obligated to see what they can get for him given that the home team doesn't really have all that much of an advantage in free agent negotiations (and if the team that trades for him does so contingent on an extension, you have to assume any offer large enough to make Lester sign on the spot is one the Red Sox won't be willing to match anyway).
It all seems like a lot of expensive hassle to dodge a couple years on a contract, but if the Red Sox have an idea that the trades involved could work out favorably for them, it's at least worth exploring.