No FA center needed.
KG won't cry if he plays center 10-20 minutes a game, more in the playoffs.
In the playoffs, KG wants to win. Celtics are good with KG at center.
By the playoffs, Melo will have improved greatly. Enough to earn 10+ minutes on the floor.
Center may be Sullinger's best position overall. Definitely defensively, especially against undersized lineups.
Sullinger is NBA ready, and can play significant minutes at center right away if needed.
KG, Sullinger, Melo, Steamer, Hollins? Wilcox? Kristic?
Celtics are stacked at center, and will be more stacked at center come playoffs because KG won't mind playing there as much, and Sullinger and Melo will be significantly improved over the course of the season. Especially Melo.
Spend the money on a stud SG-SF, and bringing back Jeff Green.