I have a very sneaky suspicion that Sully will be dealt at the trade deadline. Danny is the MASTER of selling high and Sully has played very well thus far.
On the offensive side, averaging 11 points, 9 boards and (don't discount this) 2.5 assists in just 25 minutes a night, Sully is lighting up the box score. That's 15/13/4 per 36 and those are strong numbers. Roughly 1/3rd of his rebounds are offensive and teams love that sort of stuff.
Sully has played outstanding defense this year by almost any metric. He's not a shot blocker, but his defensive rating, defensive +/- and defensive win shares are all very high. In short, he looks very solid in the middle.
Furthermore, any team that trades for him still will have the RFA right of first refusal next offseason. So a team that really wants him has an opportunity to get him now and then easily keep him next year. After all, it's not like RFAs move a whole lot (look at this past year - NONE of the top RFAs moved teams) because teams don't like tying up their cap space during the FA frenzy.
All that adds up to Sullinger's value being pretty high right now. That's when Danny tends to make his moves if he doesn't think Sully is a long-term key cog on this team. If he and Brad did think that, I think Sully would be getting more minutes.
Of course, this all depends on a number of factors. It's possible that the Cs are doing well enough that Danny doesn't want to rock the boat. There has to be a compatible trading partner and that may not happen. But all things being equal, I think it's Sully who gets shopped and moved at the deadline. Could it be for Lopez? Markiff Morris (though the way Morris is playing this year....ugh)? Perhaps even part of a deal for Cousins? Dieng? With Danny, who the heck knows?