Is there any reason to not put a player on IR if you know he's going to be out for seven+ days? You do get a healthy player in to the roster and there's the cap benefit too (probably meaningless in this case though).
There's no cap benefit to placing a player on IR, only LTIR, and even that only allows you to exceed the cap by the player's salary so it only comes into effect if you're at or near the cap. This is my basic understanding of it - our resident CBA expert Holden probably has more details than this.