Granger is coming off an injury and will be in a contract year. That gives him incentive to prove he still has it. If he has a great start to the season playing at the 3 or 4 next to Jeff Green, the team can look to trade him at the deadline. He could also be sign-and-traded. Or, if he looks good, the Celtics could sign him long term, provided he's looking for a reasonable deal. He's only 30 so he could be a useful asset moving forward if he's paid reasonably.
For the Pacers, I think Pierce would be perfect for them as a secondary scorer behind George, or even as a sixth man. The Pacers need more reliable outside shooters and somebody who can they rely on to score 1 on 1 when George doesn't have it going. I think Pierce is a better fit playing a secondary role beside Paul George than Granger. Granger is a volume scorer who would fit better on a Celtics team that will need assertive volume scorers once Pierce and KG are gone.
Granger and Green are fairly similar players, but I think each is capable of playing at either forward position on offense and defense (Green has improved in this area).