As some have said, all 5 of our starting or core line up may not be on our roster right now. The only two for sure who will start are Brown and Tatum. Horford is probably pretty likely as well. So say that is 3. The questions are who to start at PG/Guard and who to play alongside Horford.
As to PG, I have no problem starting Marcus Smart. I feel he is a good passer, good team and one-on-one defender, and a serviceable enough scorer/shooter. I also don't mind if they find more of a true PG to bring in. That is the best part of Marcus, he has great versatility.
As for the 5th starter, I went back and looked at what line ups worked best with Horford back in 2018/19. At that time, he played the most with Marcus Morris and had good results (1158 min, +74). He also had good results with Baynes (163 min, +73) and Oleleye (110 min, +46). You can also look at how he did as the one big with some combination of Irving, Smart, Brown, Hayward, and Tatum (220 min, +36). I think what this shows is that Al is very versatile also.
My prediction is:
Smart
Brown
Tatum
PF/Swing (to be acquired)
Horford
...as it seems to me Horford is most comfortable playing along side of a Marcus Morris type swing or skill PF. I think there is a slim chance this could be Jabari Parker, if we are not able to acquire someone. In Atl in 2019/20, he started 23 games with 28.6 min and put up 16.2 pts and 6.8 rebs. That would be solid production for the swing/PF role. I did not see that from him in his time with the Celtics this season, that is for sure, but maybe that is an attainable ceiling.
We could also sign and start Fournier in a 1 big line up with Horford as the principle single big. In 2018/19, they did not play that way all that much but when they did, the results were fine (see above). I prefer to find a decent PF/swing, and have that be our core line up, we can always change up and go small if we want, but it would be nice to have the option to put a PF/Swing out there with Horford.
In Philly in 2019/20, they mostly played Horford as the second big with Embiid (this was the single most frequent lineup) but they also did play a number of lineups where Horford was the single true big. In these cases, taking the other big off the court meant you were taking your best player (Embiid) off the court (not like switching Morris for Baynes) but also Horford usually had 6'-10" Simmons on the court as the second "big". The Horford-Simmons pair (without Embiid) was used a fair amount and did pretty well. Overall probably a little better than Horford-Embiid but not really appreciably so. There were very few minutes where Horford played without either Embiid or Simmons, and in those few cases, the results varied so much and there were so few minutes that you really can't do much with that data.