So that number is just wrong, on many levels.
1) It includes non-guaranteed deals -- so $17 million for Johnson and Jerebko, and $874k for Corey Walden.
2) It includes qualifying offers to restricted free agents. This is wrong for two reasons -- firstly, the cap hold is not equal to the qualifying offer (it is typically above it), and two, it includes qualifying offers to players who will clearly not be on the roster next year to get said offer (our three other camp invites other than Walden, who slot in at almost $1.1 million each.
3) It also doesn't count cap holds for 1st round draft picks, which will exist for us, but are highly variable.
Point 2 bothers me the most, because their methodology assumes no cap holds for free agents (ie you let them walk), but then turns around and assigns $10 million in qualifying offers (which will not be accepted, and again, will not equal the cap holds). And that's just to players with a chance to make the roster, never mind the $3 million to players who will be cut in training camp.
If you stick with their method and only include the players under contract (we'll pick up the option years for KO, Smart, and Young, but not the unguaranteed years of Johnsonand Jerebko), the Celtics would have about $56 million in cap room. If you add in the cap holds for draft picks, you can take away $4-7 million depending where those picks end up. So you're looking at about $50 million cap room with a roster of 12 players. You can add back in cap holds/salaries of players the Celtics want to keep at that point, because they can choose to sign/keep those players if free agency doesn't work out. But the C's are definitely top 5 in cap room. The trick is convincing the players to come.