Dani Parejo slotted home in the first half to condemn the Premier League side to defeat although victory in their final group game will still see them reach the knockout stagesSwansea City missed out on the chance to guarantee their place in the knockout stages of the Europa League after they were beaten by Valencia on Thursday evening.
By Tom Doyle
A night of rich promise turned to disappointment for Swansea as they fell to a 1-0 defeat to Valencia at the Liberty Stadium, with Daniel Parejo's first-half finish the difference between the two sides.
Michael Laudrup will feel aggrieved at seeing Alvaro Vazquez's seemingly good goal controversially chalked off just before half-time, but apart from another late effort from the striker which was cleared off the line, the Swans struggled to replicate the form they showed in their 3-0 win at the Mestalla.
A tough trip to St Gallen awaits for the Welsh side to confirm passage to the next round, and though they should still qualify, the Capital One Cup holders will view tonight as a missed opportunity to showcase their European credentials on their own turf.
The midfielder's first goal of the season earned Valencia their fourth consecutive win in the competition, with Swansea goalkeeper Gerhard Tremmel's poor clearance handing the Spaniards the game's only goal at the Liberty Stadium.
Only Michel Vorm, Alejandro Pozuelo, Wilfried Bony and Nathan Dyer remained from Swansea's last Europa League tie - a 1-1 draw with Kuban.
The returning Pablo Hernandez was named on the bench after recovering from a hip injury but Michu remained absent due to an ankle problem.
Valencia made five changes from their 2-1 defeat against Elche on Sunday, with two of those coming in the back four as Joao Pereira and Victor Ruiz started for Miroslav Djukic's side.
Bony almost put Swansea ahead when he headed just wide after Diego Alves had parried a corner to the Ivorian in a lively opening to the game.
Valencia, however, took advantage of Tremmel's clearance at the other end, as Juan Bernat's effort deflected into the path of Parejo who cleverly angled a finish home.
Jonas was next to threaten for the visitors, letting fly with a long-range effort, only for it to whistle over the crossbar, and Swansea were handed a further blow when Bony limped off injured.
The home side were further frustrated before the break, when Alvaro Vazquez looked to have levelled only for the linesman's flag to deny him, despite both he and Jonjo Shelvey, who headed the initial cross on target, appearing to be onside.
Sofiane Feghouli came close to doubling Valencia's lead four minutes after the break, shooting wide from a precise Sergio Canales throughball.
Vazquez looked to have equalised two minutes from time, only for Jeremy Mathieu to clear off the line as Valencia held on for the victory.