When GB builds a roster about the last 5 spots go to the best guys regardless of position that they dont think will make it to PS or last there very long. So, a 10-11th db, 4th TE, 6th WR, 6-7th DL, 9-10th OL, 3rd QB, 3-4th RB, 10-12th LB etc all compete amongst one other for those last few spots.
If we have no preseason injuries and a bunch of guy shine like Abby, Ringo, Joe Thomas, Hubbard, Vujonovich, Rotherram, Backman, Ripkowski , thornton, Gunter, TGW, Fanor, etc It'll be a crime to cut and LOSE a real player w/ potential for a less than JAG RB.
I'll be surprised if GB kept only 2 RBs; especially because as Beast points out Starks is not the bastion of health. But Lacy is tough we dont have to worry about him [in Det game last year, Suh broke Lacy's Jaw. Lacy took his shoe lace and wired his jaw shut and to breathe he gave himself a tracheotomy, jamming a card board core from a used roll of trainer's tape in the tracheotomy opening and missed only one play] it's possible they go with 2 on roster and 2 on PS if Neal, Crockett et al dont show much in preseason. If disaster strikes like 2013 Cincy game and Lacy and Starks go down; Kuhn, Cobb and Monty will get you thru the game until the next week when the RBs that have been practicing w/ the team all along are called up.
Lacy's asthma only acts up in the cold weather, so come Novemberish, the roster will require 3 RBs; but by that time someone will be on IR and the opening could go to 3rd RB.