Whoever plays alongside Horford in the frontcourt has to be cheap, a good defender, and a good rebounder. Think of Zaza Pachulia starting for the Warriors last year. That's what the Celtics need.
In that case you'd have 4 out of 5 players in the starting lineup able to shoot the 3. No big deal to have a guy who just rim runs, bangs down low, and cleans the glass. Plus, Horford doesn't play D and rebound like he did in his younger years.
Meanwhile, you have to make sure you're not locked into salary beyond this year, when IT is up for a new contract (and so are Bradley and Smart, if they aren't traded).
If you ask me, you go into the year with a competition for the 4/5 spot next to Horford. Think some combination of Theis, Yabusele (if he isn't stashed one more year), and a cheap FA pickup via the room exception or a minimum salary like Ilyasova.
One other reason not to go for a big splash is that end-of-game lineups will probably be smaller and have just one big on the floor (Horford). You could have a lineup of IT, Smart, Jaylen Brown, Hayward, and Horford to close out games. Besides IT, it would be a bunch of physical guys who can move their feet, switch on D, rebound etc. while also playmaking and shooting on offense. In other words, don't overspend for a frontcourt player who won't close out games for you.