Two adjustments I would make if I was MIL. First, just guard Horford one on one in post straight up. Dont foul, dont doube, just put hands up and defend. Hes not a good postplayer, hasn't been his entire career. Make him shoot his little hooks and fall aways. Doubling him just open up passing lanes. Second, play way off smart. Go under pick and rolls. Smart is a 29 percent 3pnt shooter without a splint. Play off him and all of a suddenly his play making ability is hindered and he cant shoot. This gums up celtics offense even more.
Key for the Celtics then would be if Jaylen/Jayson/Terry can create enough offense.