There is no doubt that PF is this current team's biggest weakness. I don't buy this argument that this doesn't matter. Tatum is not a PF, modern or otherwise, he is a wing (actually one of the best wings in the game) and because we don't have a real PF on the roster, we are forced to play with only one big. I don't think this is the same as saying Tatum plays PF. People say that the traditional roles don't matter anymore and then say Tatum is a PF just because he is the biggest of the 3 wings we choose to play.
There is nothing wrong with playing with 1-big, 3-wings, 1-PG at times. Most every team does this. The league MVP and the Finals MVP this year was a PF though. A good PF is still a weapon. A team is better if they have that weapon then if they don't. MIL could play Giannis in a 1-big line up if they wanted to, and they do that sometimes, but their most frequent line up is 2-bigs, for a reason.
I agree with others that I don't see Horford having enough skill to play PF alongside say RWilliams or Kanter. I think those 3 will rotate as our center, or if you prefer, our bigger big. I think people are going to be surprised at how much Parker and GWilliams are forced to play as the PF or the second big or whatever you want to call it. My guess is at least 24 min between the two in a more traditional 2-big line up. Maybe more.
Bruno Fernando is deep bench so whether you want to call him a PF or a Center or a Combo Big, it doesn't really matter. The opportunity for him is at PF though. His career numbers, meager as they are, are really not all that much different than GWill or Ojeleye. They are all about 3-4 pts, 3-4 rebs. So minutes could be there for him same as they were for Ojeleye and GWill. Parker is career about 14 pts / 6 rebs. I am sure that this is what the Celtics are hoping to get out of Parker. If that is the Parker we get, he will see 18-24 minutes at PF/Swing. If not, if in the preseason he is no better than GWill, he probably gets cut.