I think that the trade deadline happening a mere 16 hours after they got back to Foxboro from the game in Buffalo may have had something to do with not promoting Farrow. For instance, Rowe might've been a part of some "Plan B" trade thing. Shipping Rowe elsewhere would've saved cap space (maybe $500k or so), whereas putting him on IR saves nothing (probably). In order to promote Farrow, they would've had to put Rowe on IR first.
While it's true that players on IR are now "tradeable", a guy who's "dinged up" but NOT on IR would still have more value.
The ground game in Buffalo went nowhere, regardless who the Pats put out there, and I think McD and Co. were anticipating that, so they chose to experiment with Patterson out of the backfield for a number of different reasons. Seriously, watching how well the Bills closed down running lanes, even a fully healthy Michel would've had a tough go.
The Packers' run-D isn't anywhere near as stout as the Bills' is. If they're planning to focus on Brady and the Pats passing attack, running the ball, even with Barner, could be much easier.
In any case, with Rowe's spot now open, promoting Farrow seems likely, although the transaction doesn't need to happen until Sunday. I think it's likely that Michel sits another game, that Barner is the "lead back" this Sunday, and that the Pats ground game "muddles through" with 100 yds or so - just enough to "keep the Packers' defense honest". We'll see how that goes.
EDIT:
BTW - Farrow himself mentioned (on video) last week what a thrill it was to be taking handoffs from Brady. If Farrow was working with Brady and the 1st-team offense, rather than with Hoyer as part of the scout team offense against the 1st team defense, that would seem to indicate that the Pats were already prepping him for a promotion at that point.