The way I worked it out we could easily carry 4 RB's on the roster (keep in mind I don't know how the ST units are put together outside of the K, P, and LS) but it's definately possible.
Edit: Shit I forgot it's a 53 man roster, not 55. Italics denote guys I cut after the fabled 55 man roster.
Offense (in no particular order) - 25 Players
QB
J. Campbell, T. Collins, C. Brennan
RB
C. Portis, L. Betts, R. Cartwright, M. Mason
FB
M. Sellers
WR
S. Moss, ARE, J. Thrash, M. Kelly, D. Thomas, A. Mix
TE
C. Cooley, F. Davis, T. Yoder
OL
J. Jansen, C. Samuals, S. Heyer, J. Fabini, P. Kendall, R. Thomas, C. Rhinehart, C. Rabach
Defense (in no particular order) - 25 Players
DT
C. Griffin, A. Montgomery, L. Alexander, K. Golston
DE
J. Taylor, A. Carter, C. Wilson, D. Evans, E. James
LB
M. Washington, R. McIntosh, K. Campbell, M. Sinclair, L. Fletcher, H. Blades
CB
S. Springs, F. Smoot, J. Tryon, C. Rogers, L. Torrence
S
R. Doughty, C. Horton, L. Landry, K. Moore, V. Fox
K/P/LS - 3 Players
S. Shuisam, D. Brooks, E. Albright
Practice Squad
Maurice Mann (WR), B. Westbrook (CB), Nemo (FB), C. Gatewood (LB), A. Crummey (C/G), R. Jackson (DE), Martell Richardson (CB)
Possible Midseason Callbacks Due to Injury
M. Askew (DT), R. Boschetti (DT), Billy McMullen (WR), Jason Goode (TE),
So as you can see it's very possible for Mason to win a roster spot. Also in this scenario Wade and Askew got cut so those are two that could easily change some time during the year. Boschetti also gets cut.