until (if) Danny _renounces_ the Rondo (12.9M) and Prince (7.7M) TPEs AND his Bird Rights on Bass (10.3M) and Jerebko (8.6M), we are over the cap for next season.
There are potentially ways to use those tools (as well as Gerald Wallace' 10M expiring/stretchable contract) and our portfolio of draft picks and young players to facilitative acquiring LMA/Monroe/Matthews/whomever via sign & trade transactions. Thus, never actually dropping below the salary cap.
An example of the type of move you can make:
Let's say Portland wants some player who is under contract already or can be signed by some other team for 10-12M (but that other team isn't interested in keeping). Well, Danny can have the team that has the ability (Bird Rights, RFA, whatever) to sign that player do so, but trade them to Danny for a modest draft pick, absorbing the player's salary with the Rondo TPE. Now, Danny can turn around and trade that player to Portland in exchange for LMA in a sign & trade. The player's 12M contract can match up to 17M of incoming salary. So Danny just has to add some token filler (Ex.,Chris Babb), plus appropriate modest picks for compensation, in order to bring in LMA's 18.8M max contract. All without ever dropping below the cap.
The key point is that it's possible, by involving a third party, to convert the TPE into a player salary, which, unlike the TPE can both match a larger incoming salary as well as be combined with other outgoing contracts.
That's just a hypothetical, and there are lots of variations. Wallace's expiring deal is actually fairly easy to move as salary filler now (because it can be stretched, reducing cap hit) and it can be used to match up to 15M in incoming salary. Bass & Jerebko are both potential sign & trade candidates.
Few GMs are as competent at CBA wizardry as Danny Ainge is. 'No guarantee something will happen. But don't be surprised by whatever he pulls off.