The club, on Friday, released a statement regarding Juan Mata's injury stating that the player had undergone a surgery on his groin and that details will be released soon.
He has 10 goals in all competition for Manchester United.
Juan Mata becomes the 6 Manchester United star to become unavailable for the weekend game.
The injured Spain global underwent groin surgery on Friday and was forced to miss the following day's game as United were held to a frustrating goalless draw by West Brom at Old Trafford.
Mata finds himself sidelined during a crucial period for United.
Ander Herrera and Zlatan Ibrahimovic are suspended for this weekend's clash with West Bromwich Albion, and combined with Paul Pogba picking up a hamstring problem prior to the global break, it's the worse time possible for Mourinho and United to see the squad start to crack.
"I believe quite a while", Mourinho told a news conference when asked concerning the duration of Mata's likely unavailability. "That's the way we have to think, we can not be here crying or speaking about the players that are not available to play", the Portuguese said.
Without the injured trio, along with Paul Pogba (hamstring strain) and key striker Zlatan Ibrahimovic (suspended), the Portuguese coach saw the likes of Anthony Martial and Marcus Rashford fail to find a breakthrough.
"Pogba I can not say is ready for Tuesday because I don't know but Pogba is not such a long-term injury as the others".
The Red Devils face a busy final two months of the season with 11 Premier League matches still to play in their battle to finish in the top four while they remain in the Europa League.