The likelihood of McMahon landing at 2B seems very low.  It's more plausible that the club is giving him some utility reps there.  D.J. won't be traded until next off-season at the earliest and you have to have a spot for Rodgers who will probably arrive later next year.  If McMahon hits MLB pitching, a 2018 infield might look like Arenado at 3B, Story at SS, Rodgers at 2B and McMahon at 1B.

Also, Dahl is not injury-prone.  He is injured.  His other injury was the result of a nasty outfield collision that required a spleen removal.  "Injury-prone" is a term that is way over-used.  The Rox probably know by now whether he has some weird congenital rib defect, which would be extremely unlikely.  You have to count him in as a starting OF at this point.