Celtic have their work cut out to qualify for the last 16 of the Champions League as their dismal European away record continued with a 1-0 Group E defeat at Villarreal on Tuesday.
Dead-ball specialist Marcos Senna, one of Spain's Euro 2008 victors, produced a trademark free-kick 23 minutes from time to break Celtic's resistance.
Celtic now have just one point from their first two matches and their next match is a daunting trip to Old Trafford to face reigning champions Manchester United on October 21.
Villarreal are locked with Manchester United on four points at the group summit and travel to group outsiders Aalborg BK on the same day.
Senna, who starred for Spain in their triumphant march to the Euro 2008 title, said that despite a good start they mustn't let up.
“Getting the three points was the most important thing tonight but we can't relax because if you go to sleep in this competition the other teams end up above you,” said the Brazilian-born midfielder.
“We dominated the second half but had to wait for the goal. Fortunately my free-kick went in.”
Celtic coach Gordon Strachan believed that for the most part his side deserved at least a point.
“After our performance in the first 55 minutes you think: 'We belong at this level',” said a dejected Strachan.
“But after 15 minutes they started to work in an area that has been giving us a problem for a while (referring to the free-kick conceded outside the penalty box).
“It was a stunning strike but the our boys were a little bit upset about the way it came about.
“Whatever, they showed what a good side they are when they went ahead.”
Celtic's travelling vertigo continues to hold them back in Europe's flagship competition with the defeat to Villarreal meaning their away record stands at 15 defeats and one draw from their five Champions League group campaigns.
Turkish forward Nihat Kahveci, last season's top-scorer, was injured for Villarreal so Jose Llorente made his Champions League bow.
With just 90 seconds gone Llorente fired a shot on the turn just over to set the tone for the first half.
Strachan entrusted the tricky Aiden McGready and Greek target-man Georgios Samaras with the task of unlocking a mean Villarreal defence that had conceded just two goals in their first five league matches.
The Glaswegian visitors soaked up pressure and there was a lot of it with former Manchester United man Giuseppe Rossi going agonisingly close twice.
Ironically for all Villarreal's pressure it was Celtic who had the best chance just minutes before the end of the first half with Samaras pouncing on a defensive slip but Diego Lopez stood tall to block his effort.
In the second half Rossi created another chance for himself with a brilliant solo run but his left-footed shot crept wide.
Strachan admitted beforehand that a draw would have been a good result and it looked like the Scot would get his wish with Villarreal running out of ideas.
However Celtic had been conceding free-kicks in and around the area and on 67 minutes Senna made one count whipping the ball over the wall past the helpess Boruc.
It meant Celtic left El Madrigal empty-handed, just like they did in their UEFA Cup quarter-final here in 2004, and they will have to pull out all the stops to reach the last 16 for a third successive season.
For Villarreal this was their first Champions League match at home since April 2006, when they drew 0-0 with Arsenal in the semi-finals of the competition, cruelly losing 1-0 on aggregate, and they look well-equipped to go on a similar run this season.