Van Gaal victorious on Old Trafford bow
Louis van Gaal tasted victory in his first game in charge of Manchester United at Old Trafford after Marouane Fellaini came off the bench to score with virtually the last kick of the match.
Wayne Rooney had seen a first-half penalty saved in the 32nd minute after being tripped by Valencia defender Antonio Barragan.
United then opened the scoring four minutes after the restart when Darren Fletcher met Ashley Young's corner on the edge of the area and fired home via a deflection.
Valencia pressed for an equaliser, however, and got their reward in the 71st minute when Rodrigo blazed a first-time volley into the roof of David de Gea's net after Tom Cleverley's attempted clearance fell to him.
The visitors were then made to rue Pablo Piatti's miss from mere yards moments later when Fellaini secured an unlikely win with his first United goal.
The United midfielder swept the ball into an empty net in injury time, after Valencia goalkeeper Diego Alves collided with his defender Nicolas Otamendi.