The Celtics will be just $13.8 million under the luxury tax ($123.7 mil) by letting Monroe, Gibson and Larkin walk. There's not much flexibility beyond that, as Theis, Ojeleye, Yabu, Nader, and Wanamaker are accounted for by min/rookie salaries.
So to keep this band together (with Baynes and Smart), we will likely need to pay the luxury tax. We could bring them both back using their non-Bird and restricted Bird rights, respectively. If Smart earns less than his current cap hold ($13.5mil), say maybe $12mil, that leaves enough salary to give Baynes a 120% raise from last season while remaining below the apron ($129.7). We would also be in the lowest bracket of non-repeater tax, and pay no more than $7.5 million for this season.
While the FO has stated a willingness to pay the tax, the question is now "how soon?" We're going to have issues with retaining this group every season beginning next year with Kyrie, then Hayward/Horford/Jalyen, then Tatum, etc. In 3-4 years, we could be paying a Cavs-level luxury tax. One option to avoid the tax for at last one year is to offload Morris to a team that can send back less (under the cap) or no salary (trade exception), though that appears to be a glaring IF at the moment. Ainge has his hands full, but this comes with being a true contender.