Arsenal right-back Mathieu Debuchy will be out for about three months after undergoing surgery on an ankle injury.
The 29-year-old was stretchered off against Manchester City on 13 September after falling and damaging ligaments.
Gunners boss Arsene Wenger said the operation "went well" but added that Debuchy's extended absence "is a blow".
"How big it will be we will only know afterwards as it will be down to how well we replace him. We bought [Calum] Chambers for that," added Wenger.
Chambers, who signed in a £16m deal from Southampton in July, was also injured after the draw with City so Spaniard Hector Bellerin, 19, replaced Debuchy in a 2-0 Champions League defeat at Borussia Dortmund.
Also 19, Chambers returned for the Premier League win over Aston Villa last Saturday and is now set for a lengthy run in the side.
|Lille (2003-2013): 258 appearances, 16 goals|
|Newcastle (2013-2014): 46 appearances, 1 goal|
|Arsenal (2014-): Six appearances|
France international Debuchy signed for the Gunners from Newcastle during the summer and played in the Community Shield victory over City.
He followed that up with six outings, which included being sent off against Besiktas in a Champions League qualifier, before suffering his left ankle injury.