Manchester United playmaker Juan Mata has undergone groin surgery, the Premier League club have confirmed.
The Spain international has been one of United's best performers under Jose Mourinho this season, with 10 goals and six assists in 38 appearances across all competitions.
The expected length of Mata's absence has not been announced but the prospect of the 28-year-old spending a prolonged period on the sidelines adds to mounting injury worries for Mourinho's Europa League quarter-finalists.
Defensive duo Chris Smalling and Phil Jones suffered respective leg and toe injuries while on international duty with England and will be out for the "long-term", according to their manager.
Mourinho also told a news conference ahead of Saturday's game against West Brom that he had "no idea" when Paul Pogba would be back from a hamstring injury.
A brief statement released by United late on Friday read: "Juan has had surgery for a groin injury. Further updates on his recovery will follow in due course."
Top scorer Zlatan Ibrahimovic and Ander Herrera are suspended for Saturday's game against Tony Pulis' Albion, although captain Wayne Rooney is set to return from a knee problem.
United lie fifth in the Premier League, four points behind fourth-place Liverpool with two games in hand on Jurgen Klopp's side.
Anderlecht are up next in the Europa League – part of packed nine-game schedule in April for Mourinho's men.