Mount faces several weeks out after breaking down in Sunday's win at Manchester City.
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He posted to social media: "Words can't portray how devastated I'm feeling right now.
"You could probably see the look on my face when it happened. I knew what it meant.
"United fans, you may not know me too well yet but one thing I can guarantee, I will never give up or lose faith.
"I've said this before, but I will continue to give everything, get through this tough period and won't stop until that's achieved."