So the one major stumbling block I see when people are trying to create trades for veteran stars right now (Butler or George, etc.) is having to match salaries without giving up too much value and without using Zeller (because then we lose our shot at max cap space. Essentially, 90% of the trade proposals posted on here for a vet mean that we wouldn't be able to sign Hayward or Griffin and the trade would take the place of a big FA signing.
In fact, we actually need to drop a little bit more $ to get in position to offer Hayward or Griffin a full max. So, with Paul George's situation driving his price down a bit, I think we have a unique opportunity to bid on him with an offer that would entice the Pacers but still leave us with major future assets to work with.
Option 1:
Celtics Get:
Paul George ($19.3)
Pacers Get:
Avery Bradley ($8.

Jae Crowder ($6.

Marcus Smart ($4.6)
1 lesser future 1st rd pick (BOS 18, BOS 19, LAC 19, or MEM 19).
Total $: $20.2 (Celtics gain an additional $0.9 of cap space moving incrementally closer to max room)
Reasons why this trade is the top option for the Cs:
1) Allows us to keep all of our best future assets (Jaylen, #3 Pick, BRK 18, LAL/SAC Pick) for further development or a later deal.
2) Removes 2/3 players that would need to be paid next year in AB and Marcus Smart. I realize it would be cost-intensive to retain both IT4 and PG13 next off-season, but it would be technically possible with ownership's commitment to spend tax $. We would also maintain those future assets (Jaylen, #3 Pick, BRK 18, LAL/SAC Pick) on rookie-scale deals to help somewhat minimize the tax implications of paying 4 max or near-max players.
For the Pacers:
This is a pretty good haul consider their situation. They are guaranteed one starter level player on a good contract (Crowder) and one future first. They also get two more good players in Bradley and Smart. They could potentially re-sign Smart as an RFA next offseason and flip Bradley at the deadline for another late 1st (or they could re-sign Bradley as well if they like him). Their reported asking price is a starter and 2 1sts (which they'll be lucky to get), and this deal arguably gets them more than that.
Celtics Roster with Griffin:
1- Isaiah Thomas / Rozier
2- Jaylen Brown
3- Paul George / Jayson Tatum or Josh Jackson
4- Blake Griffin
5- Al Horford / Zizic
Remaining High-End Future Assets: BRK 18, LAL/SAC Pick
Celtics Roster with Hayward (Would obviously need to add size):
1- Isaiah Thomas / Rozier
2- Gordon Hayward / Jaylen Brown
3- Paul George / Jayson Tatum or Josh Jackson
4- Al Horford
5- Zizic
Remaining High-End Future Assets: BRK 18, LAL/SAC Pick
Option 2:
Celtics Get:
Paul George ($19.3)
Pacers Get:
Avery Bradley ($8.

Jae Crowder ($6.

Jaylen Brown ($5)
Total $: $20.6 (Celtics gain an additional $1.3 of cap space moving incrementally closer to max room)
This option inserts Brown in place of Smart and a future 1st. It gets the Celtics a bit more cap room, but they lose a high-end future asset in Brown.
Celtics Roster with Griffin:
1- Isaiah Thomas / Rozier
2- Marcus Smart
3- Paul George / Jayson Tatum or Josh Jackson
4- Blake Griffin
5- Al Horford / Zizic
Remaining High-End Future Assets: BRK 18, LAL/SAC Pick
Celtics Roster with Hayward (Would obviously need to add size):
1- Isaiah Thomas / Rozier
2- Gordon Hayward / Marcus Smart
3- Paul George / Jayson Tatum or Josh Jackson
4- Al Horford
5- Zizic
Remaining High-End Future Assets: BRK 18, LAL/SAC Pick
I think the 1st option in particular is a worthy gamble for the Celtics. Yes, they would have to convince Paul George to stay in FA next year, but they wouldn't be gambling any of their top 4 most valuable future assets to do so. They would also be a team coming off either a very competitive ECF berth or a Finals berth with potentially 2 top-5 draft picks coming to bolster the depth and talent. They would have the opportunity to offer the most money. I believe that would be hard for George to walk away from.