The other quality about the Rams defense, the guy that is generally the "in the box" safety, is the one who will draw the assignment of covering the TE. This is where the Rams have been very poor. Even Mikell, as effective as he was in blitz packages, and run support near the LOS, lacked the size and cover skills to stay with the hybrid TEs they face. McDonald was brought in, IMO, to address this issue directly. With McLeod thrust into the nickel role much of last year, they were forced to play either Stewart or Giordano as the single high safety, and it was noticeable. The biggest problem area with the safeties hasn't been their cover skill, its been the poor tackling.
People seem to want to insist on needing a "FS" ball hawking safety. But in this scheme, that just isn't all that important. The biggest thing that needs to, and IMO, will improve is the tackling on the back end. That is where Alexander may leapfrog the likes of McLeod and others being counted on. Even Cody Davis is a much better tackler than some of the other options. I fully expect the guy who shows they can get guys on the ground consistently, will be the one who plays come Week One.