The final playoff roster:
Pitchers (12): Breslow, Britton, Dempster, Doubront, Lackey, Lester, Morales, Peavy, Tazawa, Thornton, Uehara, Workman
Catchers (2): Ross, Saltalamacchia
Infielders (5): Boegaerts, Drew, Middlebrooks, Napoli, Pedroia
Outfielders (5): Carp, Ellsbury, Gomes, Nava, Victorino
DH (1): Ortiz
Guys on DL: Snyder, Bailey, Buchholz, Hanrahan, Miller, Wilson, Kalish
With Snyder and Buchholz being activated (or planned on being activated), that leaves:
Free Slots (5): Bailey, Hanrahan, Miller, Wilson, Kalish
So, of the guys you have read about - McDonald, Berry ... as well as the guys like Lavarnway, De La Rosa, others on the Pawtucket train ... 5 of them will make the postseason roster. I'd expect both McDonald and Berry to make the roster as guys with specific skills (glove and stolen bases). Lavarnway seems like a safe bet, since a 3rd catcher makes sense to have eligible. So two guys left who were in the organization as of yesterday who can be added to the playoff roster. Of course of the three guys just mentioned, I feel most confident about Lavarnway's inclusion.
Remember if the Red Sox do not win the division, they have to play the WC round. But the WC round also includes a roster reset before the ALDS - so the Red Sox could easily carry extra position players for the WC game (where you don't expect to need 10 pitchers, let alone 12). Could easily see Berry or McDonald added then for a one-game need.