Lackey's only obligation, contractual or moral, is that if he decides to play in 2015 for $500K, he should perform up to his physical ability and doesn't whine about it.
Now, if he wants to continue pitching and earn another contract, then he should be willing to pitch in 2015. If he takes a year off, then in the winter of 2015-16, he'll be a 37 year old pitcher who's only thrown 400 innings over the last 4 years, so any contract offer is likely to be small base with big incentives. And if he chooses to take a year off, he'll have to find a way to keep in shape on his own dime. Given how much Lackey seems to love the concept of "team," I just don't think he's the type to relish being a lone wolf. The biggest issue is if he pitches in 2015 as a pending free agent, what happens if Lackey is expected to pitch through some type of pain and to gut it out in a late season run at a playoff berth? The Sox don't/won't care about it might cause further injury, while Lackey may be risking big money while performing for peanuts?
Now he has every right to use the threat of retiring in 2015, to negotiate an extension, and the Red Sox have every right to use the minimum salary year as leverage to negotiate a team-friendly extension. So it seems unlikely that they'll enter the off season with this not resolved. And although the Sox don't appear to need any luxury tax help in the coming years, a new deal, averaging in the $500K year, could end up with a multi-year deal with a lowered AAV, enabling them to offer big pre-arb offers to players like Bogaerts. One difficulty is in determining Lackey's market value in the event of a trade.
For the record, I think Lackey is a good risk for a pitcher in his late 30's.