Boston's payroll is up there, but they will now become eligible for an disabled player exemption salary slot for Rondo. As I understand it, the exemption is worth half the player's salary or the MLE that year, whichever is less, and it can be used on multiple players.
There's not a lot out there in FA PGs (Fisher, Boykins, Bibby), but they could bring in a combo guard like Delonte West.
Along with signing someone like Delonte, they could get creative and get a jump start on the rest of the league and sign Greg Oden for this year and next. It would be ironic to sign an injured player with the disabled player exemption.