The Hoops trail 1-0 to the Gibraltan part-timers following an embarrassing showing in Tuesday evening's first-leg tie at the Victoria Stadium.
Next up for Celtic should they recover in next week's return match is a two-leg affair with either Kazakhstan's Astana or Zalgiris of Lithuania, who are locked at 0-0 following their first-leg meeting earlier this week.
Northern Irish champions Crusaders have been pitted alongside Astra Giurgiu of Romania, but they must first turn around a 3-0 scoreline against Copenhagen if their European journey is to continue.
It remains all to play for between Welsh representatives The New Saints and APOEL heading into the second leg in Nicosia next week, meanwhile, with the winner of that match - currently goalless at the midway point - to face either Rosenborg of Norway or IFK Norrkoping of Sweden.
Elsewhere, Fenerbahce will face AS Monaco and former European heavyweights Ajax are paired with PAOK for a place in the playoff round.