Amad is set for a lengthy spell on the sidelines and could potentially miss the rest of the season.
"It was a good week until two days ago when we had some problems,” said United boss Ruben Amorim on Friday.
Amid mouting injury problems at United, Chido Obi-Martin is in contention for a place in the squad after several youngsters were called into first-team training this week.
The 17-year-old has been a revelation in the United academy since joining from Arsenal in October and scored a hat-trick in the FA Youth Cup win over Chelsea in midweek.
“I don’t want to say names,” said Amorim. “We have to be careful on that. We have problems this week, we called some young players to be in our training. We have some data evaluation. He is one of them.”
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