Montgomery, Adams and Abby have all had trouble staying healthy. Note that Adams' bad 2015 allegedly stemmed from being injured last season according to some posters around here.
In addition, Montgomery has had trouble tracking the long ball every bit as much as Janis does. We witnessed that last preseason and in his shortened regular season. It was also mentioned by scouts in his pre-draft reviews. There's possibly a good reason he plays out of the slot. From what little that we've seen of him so far he has flashed good YAC and ST return potential. His speed is okay but he's not a go-to guy for the long ball. He just hasn't had enough chances to show what he's capable of doing due to that ankle injury. He seems to have upside.
Abby gets good separation though he's not really a burner. He'll beat a DB consistently with his savvy but not necessarily with his speed. His routed running is fundamentally sound, per McCarthy. He can contribute in the return game. He has upside.
Adams was rated second from the bottom for WRs last season. His YPC for a WR was simply awful in 2015 at less-than 10. He struggled mightily to get separation for most all of last season. He dropped a some passes. At one point last year he was criticized for spending too much time playing video games instead of working on his craft. There have been no reports of him still hurting from last season. He's made a couple of nice catches in the preseason games but that's what WRs are supposed to do, no? He's young and maybe maturity will help with some of what has held him back, though it won't make him run faster and his "speed" was already marginal. He must have been picked because he could bring route-running savvy to the position. We have not seen that on a consistent basis. He does not contribute on STs. For a second round pick he's been a huge disappointment, up to this point.
Considering all three above to be locks or near to it might be just a wee bit optimistic. All have had flaws in their games. Personally, it would not surprise me if any one of these three was injured, traded or cut by the end of preseason or when Janis is eligible to return (I had to throw his name in there to honor the title of this thread). This could especially be the case with Adams. Maybe the Raiders and their QB would show some interest in him. Then again, injuries (yet to happen) could play a significant role in shaping that final roster. So would stellar performances by Davis and Geronimo over the next couple of weeks. Barring injury, Cook, a slimmer R Rodgers, and a steady and dependable Perillo (or Bachman) may have some influence on how many receivers will need to be kept on the roster.
The next couple of weeks will be enjoyable to watch all this being played out. There's always a surprise or two. Maybe it comes from this position group.