Kieran Tierney will miss three months of action after Arsenal announced the defender will have surgery on his dislocated shoulder.
The former Celtic full-back, signed for a reported £25million in August, hurt his right shoulder against West Ham on Monday after falling awkwardly in his own penalty area.
Arsenal confirmed a significant blow for the 22-year-old on Saturday, saying he "will undergo surgery next week and rehabilitate for around three months".
Tierney joined the club during rehabilitation for a pelvic injury, limiting him to 11 appearances across all competitions.
In more positive news for the Gunners, Nicolas Pepe has been declared available for selection ahead of Sunday's home Premier League match with Manchester City.
Forward Pepe had suffered bruising to his left knee after scoring in the 3-1 win away at West Ham.
But Rob Holding (knee), Dani Ceballos (hamstring), Hector Bellerin (hamstring) and Granit Xhaka (concussion) are all out.
Bellerin and Ceballos are hoping to return to training next week, while Holding is expected to be back before the end of the month. Xhaka, meanwhile, will be managed under concussion protocol rules.