In 2012, this team, with B+ level injury luck ... was the best team in the game from wire to wire basically. If you evaluate the lineup changes in the offseason by position (my guess with some caveats):
LF: Same-maybe a shade lower (Nava is a regression candidate, Gomes a positive regression one, both should not be big deals)
CF: Lower (although Bradley's on-base skills could REALLY help here)
RF: Same-shade lower (age-related regression, though if the power spike coming from quitting hitting lefty is real it's a different player we are evaluating)
3B: Higher (Bogaerts or Middlebrooks for the full season will be fine)
SS: Higher (Bogaerts or Drew, former has more ceiling although latter is just fine)
2B: Higher (figure the injury healing will enable the power to return in earnest)
1B: Same (Napoli I expect a similar season, Carp is a regression candidate perhaps)
C: Slightly Lower (Pierzynski's age and on-base skills a dropoff, Ross is Ross)
DH: Same (Papi has to show me the regression first)
To the pitching staff:
Lackey, Peavy, Dempster have some age-based regression possible ... at the same time Peavy should be better than he was at the end of last season. Lester is one of the surest things in the game and Buchholz we can rattle off the wide range of outcomes. That said, overall this rotation is good and if they make 144 of 162 starts like they did last year - the rotation will be a strength.
Bullpen - Breslow is a prime regression candidate. To be fair Uehara is too only because that level of achievement is impossible to expect. That said, Workman, Britton, De La Rosa, Tazawa all have reasons to expect improvement or continued solid performance. The new guys are all solid adds, and Cherington will keep adding more bodies because that is the best approach to take to the bullpen.
I think looking at the breakdown, if a legitimately good LF option showed up, they'd really have to look at it. Otherwise, the positions are in decent shape. The bullpen is always worth adding arms - and the rotation while it lacks a true #1 (although Lester did a pretty good job as such in the playoffs) is rich in #2/#3 sort of guys, and you can (and have) win a title that way.