The winger damaged his ankle in the 2-1 home defeat to rivals City, and Sir Alex Ferguson has revealed the 27-year-old will play no further part in their title pursuit.
The winger has been in and out of the team this season with injuries, only featuring in 19 of their 33 Premier League encounters, with his last appearance coming in the 2-1 home defeat to Manchester City - in which he was withdrawn in stoppage time.
Young travelled to support the team against West Ham on crutches in midweek, with Ferguson now confirming the winger will play no further part in United's bid for a 20th Premier League title.
"Ashley is out for the season," the boss told MUTV. "It's unfortunate and it's a bad blow. We thought it'd be maybe two or three weeks but now he's been down to the specialist and that's why you saw him at the game on Wednesday."
He added, "We took him down to a specialist in London and the prognosis was not very good at all. We're going to lose him for the rest of the season."