They aren't going to crowd their rotation anymore than the have to. They need slots for 2019, but will play the 2019 season and hope that more of the pitching talent is ready to go then.
Right now the Sox are looking at Lopez, Giolito, Rodon, and Covey as starters. The need for 2019 is real. I going to guess the Sox sign one real major league starter looking for a bounceback, along with a couple of MiLB deals with invites to spring training for guys to see if they can win slots, and if not pitch in Charlotte as depth.
When 2020 rolls around they get back Kopech, and they should have some combination of Cease and Dunning knocking on the door, assuming no one else goes crazy (such as Hansen going back to 2017 form). They also have space fillers like Adams, Stephens, Fulmer, Danish, and Guerrero floating around if they need a filler arm, or as low likelihood breakouts.
If they get to the 2019 off season and this isn't in place, there will be more free agent pitchers to pick and choose from. There is no good reason to block any more pitching slots than you have to with the pitching the Sox have in the pipeline. Go year by year and see what you need for the short term.