The Ecuador winger headed United into the lead in first-half injury time and, after Robin van Persie equalized in the 71st, darted into the area with nine minutes left to give Danny Welbeck the chance to smash the ball past goalkeeper Wojciech Szczesny.
The result kept the defending champions within three points of City and further threatened the Gunners’ chances of qualifying for next season’s Champions League.
While Thierry Henry watched from the stands having been left out by Arsenal, Ryan Giggs showed the difference a veteran can make when he crossed for Valencia’s first goal.
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