I think with roster as is you retain KO and play Horford at the 4. Tatum at the 4 off the bench is okay. Depending on how he does there may tell you what to do the following year.
IT, AB, Crowder, Horford, KO
Rozier, Smart, Brown, Tatum, Zizic
Nader, Yab, Semi, Theis
I think if you are going to add Hayward you have to look at Crowder and AB for a PF with Brown moving to starting SG.
IT, Brown, Hayward, X, Horford
Rozier, Smart, Tatum, Yab, Zizic
Nader, Semi, Theis, X