It depends. If the team is a contender (or has an obvious and immediate path to contending that doesn't involve free agency), then you give him a max contract (maybe not for 5 years, though). If the team strikes out on trades and free agency this year, and looks like they'll be better off by building through the draft, you probably let him go in free agency (or sign him to a shorter-term max contract, probably 2 years so he and Horford expire at the same time)
I don't know how things are gonna work out, but I really hope it's option 1