Let's assume for a minute that management wants somebody other than Tatum as our starting PF, and that Horford's best role at this stage is as a backup big man.
Who are the power forwards that 1) are potentially available, and 2) are good enough to start?
John Collins is the obvious one. He's overpaid relative to his production, and he'd cost one of Smart / Brogdon / White. But, he's capable of playing the position. Smart + Kornet for Collins? I'd do it.
Other options?