The 21-year-old winger has suffered ankle ligament damage and is expected to undergo surgery, with United having few forward alternatives.
Amad has been one of United’s standout performers, tallying nine goals and six assists, making his absence a significant blow.
Amorim, already struggling with a depleted attack, now faces the challenge of reshuffling his options with limited firepower.
The manager had hoped to strengthen his frontline in January, but didn’t get any players, while loaning out Marcus Rashford to Aston Villa and Antony to Real Betis.
Prior to Antony going to Betis, Amorim stated: "I want to keep my players. I'm focused on this, these players. I don't know what is going to happen.
“This window, we'll see, but he did a good job (against Southampton)."
If United's deal with Betis includes a recall option, Premier League rules state Antony cannot be called back until February 22.