Honestly, I don't know how teams go about determining if a player is prone to injury, or whether it just so happens that they got injured two or three times in succession. At some point, you just can't reserve a roster spot any longer, but, that's what IR is for, anyway.
Personally, I'd hate to see us cut Johnson. I know the team was very excited about him on draft day, and it's very clear by their attempt to keep him active last season that they thought he could contribute on game days given what they'd seen in practice. This year, I liked a lot what I saw from his limited preseason snaps. I'm hoping against hope he's able to practice soon. If he's able to get on the field before the first week of the season, I'd have him on the roster. Then, if he's hurt, IR him. If it's niggling injuries the first half of the year or so, I'd consider cutting him then and bringing Heath up from the PS to take his place. Though I'd understand if the team were to move on at this point if he's not ready for the regular season.