They'll have one eye on the £40m at stake for winning the league this year and getting straight into the CL group stages next year. Not that i'm using that as an excuse as they weren't good enough to get past a Norwegian team who not many had heard of a few months ago, who are in pre-season and recently sold 4 of their best players.
Celtic have been embarrassing in Europe the past 4 or 5 years having been pretty good and tough to beat (at home especially) before that.
Show's how much work they still have to do if they're to get back anywhere near that level and even avoid finishing bottom of a CL group.