Pick three pitchers from this list:
Buchholz, Bumgarner, Cahill, Cashner, Chacin, Cole, Eovaldi, Estrada, Fiers, Gibson, G. Gonzalez, Graveman, S. Gray, Happ, Harvey, Hellickson, E. Jackson, Keuchel, Lynn, Miley, S. Miller, Morton, Nova, Odorizzi, Pineda, Pomeranz, Porcello, Roark, Ross, Ryu, Sabathia, Sale, An. Sanchez, Santana, Shields, Shoemaker, Verlander, Wacha, Wheeler, Wood
The Sox could make a trade, but it probably won't be for anyone substantial without giving up Benintendi or Devers. Unless Darwinzon Hernandez continues to break out and is somehow ready in 2020 (it's possible), we need to sign or re-sign three from this list. The bold ones are available now, the others next year. Probably half of them should never pitch for the Red Sox.
Without signing Eovaldi, that task seems extraordinarily difficult.