The issue is that one injury in warmups will make them have to play with 7 D in the season opener. The issue is not having a spare forward in case of an injury, not who the 7th defenseman is.
Again, it's not a big deal because they can just waive Shea tomorrow and have a 13th forward for the remaining 81 games. But it seems like an unforced error for the first game of the season that could have easily been solved by just waiving Shea like they did with Zohorna, I don't really understand why they didn't do that.