The Premier League club announced this via their official Twitter account on Friday night.
“Juan has had surgery for a groin injury. Further updates on his recovery will follow in due course,” the short statement read.
Mata’s injury is a huge blow to manager Jose Mourinho, who lost defender Phil Jones and Chris Smalling to “long-term injuries” during the international break.
Mourinho will also be without Paul Pogba, Ander Herrera and Zlatan Ibrahimovic, when they host West Brom in the league this weekend.
When confirming the injury lay-offs for Smalling and Jones on Friday, Mourinho said: “We cannot be here crying or speaking about the players who are not available to play.
“You know Zlatan and Herrera are suspended, you know that Herrera is two matches banned for that episode against Chelsea.
“You know what happened in the English national team with Smalling and Jones. You know that we have also Pogba injured, but I repeat they’re not important.
“The important players are the ones that are ready to play and the ones I trust and the ones who are going to give us everything tomorrow to try to keep us in the fight for the top four.”
United have a packed schedule in April, with nine games spread across domestic and European competitions.