A reasonable expectation is far from a guarantee.
It is a reasonable expectation that OBJ can be ready for the final month of the regular season. However, considering what is at stake, there is no guarantee. The Rams made a huge move to obtain Robinson who is a dominant WR. Woods, although versatile and a very good WR, was not a dominant WR. The Rams now have 2 dominant WRs, a 3rd that is highly skilled and reliable (and young) and some up and comers.
OBJ would be a good addition ONLY if he is 100% or something near it. An 85% OBJ isn't good enough.