It depends on who the other guys are, but Thomas and next year's Brooklyn pick (by which I assume he means 2018) is a good start. The '18 Nets pick will probably not be as good as '17 and you are getting a premier player to pair with Horford, who is essentially just going to decline, so in some ways they have to realize some short-term goals. Getting a chance at a player like that, plus being in the driver's seat for the contract you can offer him, is definitely worth Thomas and a first-round pick. The Celtics will also have to contend with who they will pay as these young guys come up, so consolidating into 1 premier player makes the most sense, considering that Boston would still have the extra '17 first-round and 2 extra first-rounders in '19.