Horford is at absolute best a 3rd best player type player at this point, probably 4th, but the Sixers knew that when they signed him and that is what they signed him to be. He is overpaid at ~25% of the cap, but you often have to overpay to land a free agent from another team. The problem is, Horford quite simply can't play power forward for any extended period of time, and that is what they were signing him to be. That isn't a Horford problem, that is a new GM that is in overhead and doesn't know how to properly construct a roster problem along with a coach that seemingly can't figure out what to do with his 2 best players problem.
Games like last night would come a lot more frequently if Horford was playing center consistently as he has been doing the last 6 games (and he has generally played better in them overall). The Sixers are 3-3 in the last 6, not only without Embiid, but also without Simmons. If Simmons was out there, I'd expect them to be 4-2 (5-1 would be difficult, but they certainly could have beaten the Clippers even without Simmons). Horford doesn't fit well with Embiid, but the reality is the Sixers signed him mostly to fill in the gaps when Embiid was out (both in a game and for whole games). The Sixers on the whole haven't been the complete and utter trainwreck this year when Embiid has missed games, and that is largely a function of Horford. He has served that main function. He just hasn't been very good playing with Embiid and frankly I'm not sure they would have been a fit even if Horford was 28. Horford has quite simply always been better playing center no matter how much he dislikes it.
If the Sixers get healthy, their best starting 5 is Embiid, Harris, Milton, Richardson, and Simmons. They need the shooters around the 2 stars and Milton has been playing so well, I think you need him to start. Horford being the bench anchor with Robinson, Scott, Burks, and Korkmaz/Thybulle makes a lot of sense. And they should still have lineups with Embiid and Horford on the floor together in the middle of the 1st and 3rd quarters so that they both end up in the 28-32 minute range every game (obviously playoffs that might adjust some). Brown was given a poorly constructed roster, but he hasn't coached well with it. Robinson and Burks and the rise of Milton should make it easier for him to maximize Embiid and Simmons though, they just haven't been healthy since the new additions.