It's small but it's the way the NBA is going. Tatum is basically a 4 in today's NBA. Plus in 2018 you have a chance to draft another big and bring over Yabusele.
In the meantime, it's possible to try to use the exception to get a PF like Luc Mbah Moute, Patrick Patterson, Jamychael Green, or James Johnson. Hopefully one of those guys could be had for cheap.