I think you're underestimating him.
As a young center who averaged 16 and 11 per 36 and .123 WS/48 this season (Sully was 17 and 11 per 36, but with much lower efficiency, and .93 WS/48), Mozgov is also 7'1" (a good 4 inches taller than Sully).
And he's on a good contract for the next two years. Why would Denver want to give him up?