You do realize that after Gerrit Cole, the Yankee rotation is not much better than Boston's.
Montgomery at #2 "was a solid piece once." So "used to be good" is a justification here? First of all, he was an OK pitcher, and that was over 3 years (and 75 IP) ago. He is essentially a cheaper version of Nathan Eovaldi, but somehow less durable. And worse.
German is OK as a pitcher, but let's not go overboard. He's never thrown more than 143 IP in a season or been worth more than 2.0 fWAR. In both of these respects, he is less than Nick Pivetta, but, German is older.
Deivi Garcia had a nice 4 starts. So did Tanner Houck. Even if you like Garcia's pedigree more, and you probably should over the never-ranked Houck, the future is still equally unknown for both.
Oh, and that leaves ERod vs Clarke Schmidt or whoever. Advantage: Boston. Do not even try to counter that one. Ditto the same when each team gets back their mid-season pitchers. (Severino vs. Sale.)
Now, the Sox are unlikely to add a pitcher as good as Cole this season (unless you count Sale), but Gerrit is the only thing that separates the Yankee rotation from the rotation of a team you recently said "as presently constituted [was] a last place team."