Los Che are in a clutch of sides pushing for a possible spot in either the Europa League or Europa Conference League in 2022/23, but they are now cut 13 points adrift of the top six.
The hosts carved out a string of half chances in the first 45 minutes but they were unable to find a genuine breakthrough at the Estadio Mestalla.
That lack of a cutting edge allowed Osasuna to force themselves in front in the contest after the restart as Chimy Avila fired home from the penalty spot.
Chimy Avila makes no mistake from the spot! 👏
Osasuna lead after Valencia were punished for a hand ball 🖐️ pic.twitter.com/5BbpiHTCA4
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FT #ValenciaOsasuna 1-2 🔴
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