Celtic’s entry to this year’s competition, only confirmed on Wednesday when Swiss club FC Sion’s UEFA ban for using ineligible players was confirmed, started disastrously.
In the second minute Diego’s cross was met by Colombian striker Radamel Falcao and he bulleted his header from 10 yards past Hoops goalkeeper Fraser Forster.
Gary Hooper could have equalised soon afterwards, but dallied on the ball and the chance passed.
After spending much of the first period in their own half, Celtic made attempts to take the game to the Spaniards after the interval. Charlie Mulgrew’s free kick from 20 yards out on 66 minutes was the closest they got to a goal.
Two minutes later, Celtic were punished for slack marking in the area when Diego coolly first-timed a cut-back from the byeline to beat Forster with ease.
Watch video highlights below: