Indiana has budget constraints, and Lance won't fit into their current budget with the money he's being offered. They need to shed a little payroll:
Pacers get:
Keith Bogans
Draft rights to Colton Iverson
Celtics get:
Ian Mahinmi
Lottery protected 1st round pick, presumably 2015, but I'm okay with 2017, since we have so many picks the next two drafts.
Mahinmi has use as a center, and isn't signed to a terribly unreasonable contract. That said, he got very little run in the postseason (12.7 minutes per game) despite Hibbert's struggles. This clears up a substantial amount of money for Indy to offer to Lance. Iverson is nothing special, and might not even make the Celtics roster, but this allows Indy to take a flyer on him.