Well, I don't know that I consider one relief appearance toward the end of the season as "losing his spot in the rotation", especially when you consider how Gibson was over his last six starts. Had the season gone another few weeks I have a strong feeling we would have seen Syndergaard back on the bump to open a few games.
Point being, Syndergaard, Falter, and even Gibson and his 9.73 ERA are all still considered starting pitchers for the time being. That bottom of the rotation starters would be on short leashes in postseason starts is nothing really new, right?