Well, we now have an explanation as to why Nylander was absent from the event. According to The Hockey News, he and his brother were at a golf tournament in Stockholm, Sweden. They were participating in the apparel brand GOATLANE's official launch, which was taking place at the Bro Hof Slott Golf Club.
This is an event that had been planned weeks in advance, and there was no way the Nylanders could have known about the plans to make Matthews their captain. If he had been in Toronto, we're sure he would have made the effort to go. Having said that, Leafs fans can now breathe a bit easier about the locker room situation.