Course it's one off. As I say, when you play against Ross County, St Johnstone, St Mirren, Killie, Motherwell etc then have to face European opposition - on your current budget and size of playing squad - it's easy enough for you to treat the European games all like finals. If your lowest routine league opposition was of the standard of Ipswich, Leicester, Southampton that just wouldn't be possible.
If Rangers started this current season in the english PL in their current state they'd be both relegated and out of Europe by the end of the group stage, imo. Aberdeen a huge mile further behind.