I think that's right. The Patriots.com roster seems to update pretty quickly for any transactions. It currently shows 52 on the active roster and still has Lewis on PUP, so he's not on the roster at the moment.
The possibilities would seem to be (1) they will add Lewis on Saturday and then have the option to put him on the game day roster, (2) they will sign a free agent, (3) they will elevate a practice squad player, (4) they will sign another team's practice squad player to their 53 man, or (5) they intend to roll into a game weekend with a 52-man roster. I would think of these options, number 1 seems much more probable than the others.
Edit: I suppose they could also theoretically move up Vollmer or Jackson, but I seriously doubt they'd be making room on the 53-man for either of those guys without returning them to practice first.