But they've had Cobb in the backfield and no RB several times already. Usually in the no huddle change of pace series.
RB rushing attempt (excluding QB, FB, WR, TE) averages per game in recent seasons, with the #1 back noted:
22 in 2015 (Lacy)
22 in 2014 (Lacy)
25 in 2013 (Lacy)
21 in 2012 (Alex Green)
18 in 2011 (Grant / Starks)
16 in 2010 (Brandon Jackson)
McCarthy's avowed approach in recent years is run/pass balance. The aerial circus of 2011 proved too one dimensional.
The high number in 2013 was a result of Rodgers going down for 7 games. The low count in 2010 was the result of having a 3rd. down back trying to play the #1 role and Starks being out for all but 3 games in the regular season. Kuhn served as the #2 running back that season, carrying the ball 84 times (not counted in the season average above).
You can figure McCarthy is looking for something on the order of 22 - 25 carries out of the RB position. Perhaps closer to the high end since Ripkowski did not carry the ball even once in preseason; it looks like they'll be going with Lacy and Starks for short yardage unless there's an unexpected switch to Squirewell who got 2 carries for 3 and 7 yards against Oakland. Squirewell is 5.0 guy so he'd not provide what Ripkowski does on kick coverage.
So you could see a 38/25/2 in a pass/run/gadget mix in a typical 65 offensive snap game, with little or nothing going to the FB position.
Lacy's carries per game have dropped each season from 18 to 15 to 12. As a big back who takes a lot of punishment without special endurance, the midpoint at 15 seems reasonable. You want reasonably fresh legs come playoff time. That means the #2 could go as high as 10 carries per game if you don't carry a #3. 8 would be reasonable number for the #2 assuming no injuries. Starks is being paid what many fans believe is an unreasonable amount. You can expect they'll use him for a few possessions per game.
That brings us to injuries. If Lacy or Starks were to go down in the first half of a game that leaves a hole. And it goes beyond just running the ball. While Cobb, or more likely Montgomery, could pick up some of the slack with a few carries, these guys are not trained to pass block, which is an issue.