Has to be Horford.
I wouldn't be surprised if it takes until the Allstar break for Hayward to look good and in rhythm. Next year is when he returns to his former level.
Horford is a nightmare matchup and defensive stalwart, but Davis replaces that and much, much more.
Since we're speculating that this won't happen until next season, we could always then pick Horford back up the following off season in a diminished deal, as he'll be in his mid 30s.
You can't pass up a lineup of stars like Irving-Brown-Hayward-Tatum-Davis, all in their 20s.
But we all know, it'd be Brown, Rozier and Smart that'd be on the table for AD.