Or, they could look to a trade target. Miguel Montero of the Diamondbacks has been mentioned, so let's talk about him briefly.
Montero is the same age as Martin -- five months younger. He's been consistently a better hitter than Martin, though not his equal as a defender. One thing he does well is draw walks; he's got a career .342 on-base percentage, not too far short of Martin's .354 and better than Castillo's .324.
Montero's numbers fell off a bit in 2014 and that could be because he caught 131 games, starting 130 of them. I'd think the Cubs would want a backup who could start more often than Tuffy Gosewisch and Bobby Wilson, Montero's backups in Arizona in 2014.
The obvious deal here would be to swap Edwin Jackson's bad contract for Montero's, presuming the Diamondbacks would like to save a little money and they'd be willing to take Jackson back. He pitched for them in 2010, and though he didn't post good overall numbers (5.16 ERA, 1.496 WHIP in 21 starts), he did throw a no-hitter for them. Montero is on the hook for $40 million over the next three years, while Jackson's deal calls for $22 million over the next two. In terms of a budget hit for 2015 only, it's pretty much a wash -- Montero is owed $12 million in 2015, Jackson $11 million.
With Montero being a lefthanded hitter, the Cubs could keep Castillo as his backup and thus have one catcher who hits from each side. Again, this would result in a defensive hit behind the plate, as neither Castillo nor Montero is seen as a good defensive catcher. Would the upgrade in offense be worth it?
This would also be a way of shedding Jackson's contract, and not blocking Kyle Schwarber if Schwarber can develop as a catcher; he'd presumably be ready late in the 2017 season and could take over as a starter in 2018, when Montero's deal is over.
Would you do this deal? I think I would.