Takehiro Tomiyasu will miss the remainder of Arsenal's push for the Premier League title due to a knee injury.
The Japan international was withdrawn in the opening exchanges of the Gunners' Europa League clash with Sporting CP last week, with Mikel Arteta's side losing that game on penalties following a 1-1 draw at Emirates Stadium.
Following Arsenal's 4-1 victory over Crystal Palace on Sunday, Arteta conceded the situation was "not looking good" for the defender.
It has now been confirmed that Tomiyasu will be out for the rest of the campaign after the 24-year-old underwent surgery.
A statement from the club read: "Following his substitution during last Thursday's Europa League match, subsequent assessments have confirmed that Takehiro Tomiyasu sustained a significant injury to his right knee.
"Tomi has had successful surgery in London on Tuesday and will be ruled out for the remainder of the season.
"Everyone at the club will now be working hard with Tomi, so he can join pre-season training ahead of next season."
Tomiyasu featured in 21 matches across all competitions this season for Arsenal, who are hunting their first Premier League title since the 2003-04 season, though just six were as a starter, with Ben White establishing himself as first-choice on the right side.
The Gunners hold an eight-point lead over Manchester City, who have a game in hand, with 10 matches to play.