My understanding, 12 players currently under contract for a total of approximately $100,968,558 (I say approx because the exact number for Hayward is not known). The Cap is $99,093,000. That means we have to shed $1,875,558. That part, pretty simple.
Mickey is at $1,471,382 and Jackson is at $1,384,750 so it appears to me that those two alone could get us there. [OK, some others have explained that there are rookie roster holds involved but my main point is that we need to trade for a big, not just to clear $300,000]
Problem is that we need a roster. Right now, our signed bigs are Horford, Zizic, and Mickey so if we waive Mickey, we are down to just Horford and Zizic. I think further (I am sure that if I am wrong, it will be corrected) that we can sign rookies and go over the cap; that could be Yabusele and Semi Ojeleye. I guess Daniel Theis could be signed to a minimum contract.
So if we waive Mickey (along with Jackson), and do nothing else, our big man rotation looks like Horford, Zizic, Yabusele, Okeleye, Theis., and some scrap heap min vet. Not exactly a competitive big rotation.
The point is that we need to make a trade to get a decent big no matter what. It is not just about making room for Hayward. Crowder and a pick or something like that need to go out and we need to get back a starting caliber big man.