The injury, which saw the attacker damage his ankle ligaments, has left him using a crutch and wearing a protective boot on his left ankle.
The news comes as a major blow to United manager Ruben Amorim, whose team is already struggling with limited attacking options.
Amad had been a key player for the Red Devils, contributing 9 goals and 7 assists before his injury. His absence is expected to leave a significant gap in United's offensive lineup.
Despite the grim news, Amad showed his support for his team by attending the FA Cup fifth-round match against Fulham at Old Trafford.
Initially, United's medical team provided an update suggesting that Amad would be sidelined for "a number of weeks," but it has now been confirmed that he will not feature again this season.
Amorim has expressed his disappointment at the injury, with United's attacking depth now further depleted ahead of a crucial stretch of the season.
"I'm extremely disappointed to be writing this message in such a crucial time of the season," Diallo stated on Instagram.
"Unfortunately, I will be out for some time with an injury. I will come back stronger than ever!! Time to support the boys from the outside. Still loads to play for."