So quick reset of the Celtics. they now have 13 players under contract. Assume they fill a 14th roster spot with a vet min guy and that would put them a little over 17.3 million into the tax. If they dumped Hauser+Niang that's 18.3 million, but they'd still have to fill those roster spots with minimums which would put them back over the tax. Basically, if they are trying to get under the tax there's no real way to do so without trading Simons (assuming White/Brown are staying).
Assuming all they wanted to do was bring back Kornet and Horford on minimums to fill out their roster, that would still push them back over the 2nd apron (just barely). Of course they'd still have until the trade deadline to get back under.
So all of this leads to two conclusions. If they just want to stay under the 2nd apron they'll probably dump Niang somewhere to open up enough to bring back Kornet and/or Horford on something more than the minimum. If they want to get under the tax it will probably involve Simons and Niang being traded for less salary coming back.