I'm a believer that the more reliably the starters go deep into games, the better a bullpen's potential to live up to its full potential. I was looking through Fangraphs articles to try to substantiate what is really a gut feeling of how the stats 'should be' but I didn't find anything that IMO adequately tested the proposition (i.e. something that compared a bullpen's expected performance before a season to its actual performance regressed by overall usage).
What does stand out, however, is the fact that the Sox have 4 starters among the 11 in MLB who averaged 6.5 IP/start last year. Sale posted his 5th straight such season and his 7.08 IP/GS was 0.02 the MLB leader (Kershaw), whilst Porcello was 4th in MLB at 6.76 IP/GS (behind Cueto). Price was 9th at 6.57 IP/GS which the pessimists would say is his lowest mark in 7 years but the optimists would point out was his 7th straight season above the 6.50 threshold. There were some nobodies (guys named Kluber, Scherzer, Verlander and Bumgarner) between Porcello and Price in the rankings. And rounding out the MLB Top Ten was Steven Wright with 6.53 IP/GS in his 24 starts.
Sale, Wright and Porcello were 1st (6), tied-3rd (4) and tied-5th (3) respectively in complete games as well (Cueto was 2nd and Bumgarner tied 3rd, with Kershaw, Kluber and Nova tied with Porcello for 5th). Boston + Sale had 15 complete games last year which was 3x as many as any other club save SF (10) had all year. In fact, only one other club (2014 Indians) had double-digit CGs (11) in the past 5 seasons (though 5 clubs led by Phi with 18 reached double-digits in 2011).
I don't have the resources to see how much bullpen performance improves over an expected baseline, but I did a quick set of regressions to see how a team's staters IP/GS relates to its bullpen performance. Looking at the team-level data for the past 10 years (i.e. 300 teams in all) there is a significant correlation between IP/GS and at least the nominal measures of performance (I was using mlb.com's stats and so did not have advance metrics available).
For each extra IP averaged by a club's starters (which ranged from a high of 6.57 IP/GS for the 2011 Phillies to a low of 4.72 IP/GS for the 2012 Rockies) the clubs' bullpen posted .013 lower OB%a (-3.28 t-stat), .030 lower SLG (-4.89 t-stat), 0.516 lower ERA (-4.14 t-stat) and 0.081 lower WHIP (-3.57 t-stat). What's interesting though is that neither K/9IP nor BB/9IP were statistically significant (though K/9IP, which averaged an increase of 0.34, was close with a t-stat of 1.85). Could this be because K-rate and BB-rate are more closely related to the reliever's innate ability and the other performance stats are improved by more because the pen is better rested? Not sure I can make that claim but it does point to an area of further study.
The takeaway for the Sox? If Price comes back healthy and stems his decline in IP, if Wright provides solid innings-eating through most of the season, if Porcello proves last year wasn't an aberration and continues to be Mr Reliable all year long again and if Sale is Sale and Pomeranz and/or ERod do their part, this rotation is in position to surpass the 6.5 IP/GS threshold not just for individuals but for the rotation as a whole. Only the 2011 Phillies (6.57) and 2011 Rays(!) (6.53) have done this in the past 10 years and only they, the 2010 and 2012 Phillies and the 2011 Dodgers have been over 6 1/3. The dividend that would result is a bullpen that remains basically rested throughout the season and would allow plenty of options for usage patterns that could only help.
The takeaway is that this rotation has the potential to give the Sox' Bullpen a significant boost if they can match