Very dynamic situation.
1. Who do the Celts prefer in a vacuum -- Baynes or Morris? Baynes gives that frontcourt strength, but we also have Theis returning and now Williams in the mix. Do you need more than 3 centers in a trending small NBA? Morris gives you some shooting and defensive switchability at the 4, but his role will be chopped with Hayward's return and he's only here for one more year although we could resign him with bird rights without an extra season of luxury tax implications as opposed to Baynes (we'll be going into tax already next offseason). If the answer to that is Morris, than Baynes is gone.
2. If we prefer Baynes, can we trade Morris to make it happen? Since we cannot take salary back in a trade for him, what teams under the cap or with an exception have interest in him? Maybe not too difficult, but a limitation of some degree.
3. Even if trading away Morris, will Smart + Baynes fit under the tax? Amir Johnson got 1 year $11 mil when he left Boston in a very similar role. Are we sure a team like Brooklyn who wants to save cap space for 2019 won't offer Baynes something like that? Or more? If Philly doesn't land a max contract, they would certainly offer him something like that just to keep him away from us I'd think. Basically, I'm not sure Baynes is going to accept this $5-8M figure everyone assumes he's going to take. Lots of teams with money (ATL, BKN, DAL, LAC if Jordan leaves, LAL, PHI) could use a center.
Since all 3 of those things have to be true/work out for Baynes to come back, I'd say the likelihood of him returning is less than 50%.