Given how much interest there was in him as an UDFA, I doubt Foster would make it through waivers. If we want to keep him, we'll probably need to find a spot for him on the 53. I think room can be made for him at Dobson's expense, but that would mean going into the season with only three healthy WRs (4 if Mitchell is good to go) and running a lot of 2TE sets. This all assumes that Amendola is close enough to being healthy that they won't leave him on PUP going into the season. If not, then Dobson gets his spot and this issue is pretty much resolved.
Foster could also beat out Gaffney (unlikely, IMO, as Gaffney is exactly what we need behind Blount), Develin (unlikely, IMO, as he adds too much versatility to the offense) or Bolden (more likely, IMO, but he'd have to beat out Bolden's ST value). If the Pats are adamant about keeping only 5 RBs, I think Foster's best shot is to beat out Bolden and stick as RB5 along with Blount, Gaffney, White, and Develin. To do that, he probably has to bring some ST value to the table.