With this bullpen and the offense coming around, the trait we need in a starter in this rotation more than anything else is the ability to get deep into games.
Unfortunately, it's really only elite pitchers who go deep into games. The idea of a middle of the road innings eater sounds nice, but there aren't a whole lot of them out there.
Of the top 50 pitchers by innings pitched in all of MLB, 14 have ERAs north of 4. They are:
Jeff Samardzija - 4.02 (4th in IP)
Anibel Sanchez - 4.63 (15th)
Phil Hughes - 4.32 (20th)
James Shields - 4.01 (20th)
RA Dickey - 4.87 (24th)
Mike Leake - 4.08 (24th)
Collin McHugh - 4.50 (28th)
Colby Lewis - 4.77 (36th)
RBDL - 5.06 (39th)
Andrew Cashner - 4.10 (43rd)
Jimmy Nelson - 4.21 (44th)
Julio Teheran - 4.56 (45th)
Carlos Carrasco - 4.07 (46th)
Kyle Lohse - 6.17 (48th)
By the time you get to Carrasco, you're getting 108 innings out of 18 starts (Lohse is actually 19 starts, which makes him look suckier), which is exactly 6 innings per, which I think we'd all take with Johnson.
Buch is the only Boston starter in the top 50, at 32nd, with 113.1 IP.
So, the question would seem to be: Is Brian Johnson as good as Collin McHugh, Jimmy Nelson, Julio Teheran, and Carlos Carrasco? The Sox need him to be.