inter tatters said:
The defense behind him, quite simply.
The Tigers' defense has been the worst in the Majors in Porcello's time in the league. His BABIP went down dramatically last year, due to an upswing in the Tigers defence, that should be even better here.
Dramatically? In 2012 he was unlucky (344 BABIP) but other years he ranged from .307-.316; that's only slightly high. Last year he had a .299 BABIP, the Sox shouldn't be expecting that number to drop much further as pitcher BABIPs tend to range from .290-.310.
Porcello has improved but his greatest asset is his durability while providing average production. The trade makes sense but isn't a great win. He's a 3.4-3.75 ERA starter based upon his FIP; that's good for about 40-50th best.
Edit: Let me add some for context to that last sentence...
In 2014, 120 pitchers pitched more than 130 innings. There are 150 "starting" spots in MLB. So, each team had ~4 "starters".
The breakdown on ERA was:
25th best - 2.93
50th - 3.46
Top 75 - 3.72
Top 100 - 4.39
119th - 5.44
So, based on STEAMER projections for Buchholz, Kelly, Miley, the Red Sox have approximately the ~90th, 110th and ~80th best starting pitchers.