Well that's they key, right? IF McDonald is healthy enough to be active, and IF Mankins can dress despite not practicing.
Let's be clear: I'll be thrilled if the Pats don't need to "call up" an extra OL this week.
They've had one inactive OL each week except one. If McDonald can go and Mankins can't, that's what they've been dressing. 7 OL gives them two backups, Cannon for tackle or RG, and McDonald for an interior spot. If McDonald can't go (I'm assuming Mankins is out), I agree they'll need another OL, probably Warren or Austin from the practice squad. I think McDonald can go, though. He was limited in practice all week 2 with a "Shoulder" but dressed with Connolly suspect (Connolly ended up dressing but not playing). With Connolly better last week he was inactive. Maybe his shoulder got worse, but I think him being inactive had more to do with the depth on the OL.
As for this, I don't think what Slater has done in the past tells you a lot about what Slater might do on Sunday. Going into this year, did you expect Edelman would play over Welker in 2 WR sets?
Given Slater's enormous ST role, I would be really surprised if Belichick activated someone else to be the 4th WR who (as far as I know) has no versatility on ST and a limited knowledge of the offense.
I'm certainly not suggesting Slater's going to be inactive. I just think they are short a skill position player. Last week they dressed 4 TE and 5 WR (including Slater), and we know two of those guys (Edelman and Winslow) aren't going to be active this week. Salas dressed week one, so I don't think it's so far-fetched that he'd be on the 46-man roster, especially since he's been in the practices with the team.
One more thing we haven't talked about: with Edelman out, Welker likely becomes the primary punt returner. That's another reason I'd want an extra body at the WR position.