Arsenal right-back Hector Bellerin has been ruled out for six to nine months after rupturing his anterior cruciate ligament.
Bellerin sustained the injury during the second half of Arsenal's 2-0 win against Chelsea on Saturday when he went down unchallenged and was carried off on a stretcher.
The 23-year-old was making his first start since mid-December having overcome a calf problem but will now miss the remainder of the season, and possibly the start of the 2019/20 campaign.
Arsenal revealed the severity of Bellerin's injury on Tuesday morning with a statement on their website.
"Further to the injury sustained during our match against Chelsea on Saturday, we can confirm that Hector has ruptured the anterior cruciate ligament of his left knee," it read.
"Hector will undergo surgery to repair this in the coming days. The rehabilitation process is expected to take between six to nine months and therefore rules Hector out of action for the remainder of this season.
"Everyone at the club will now be working as hard as we can to ensure Hector is back on the pitch as soon as possible next season."
While the January transfer window would allow Arsenal to bring in cover at right-back, Emery does have options in Stephan Lichtsteiner, Ainsley Maitland-Niles and Carl Jenkinson - who was expected to be allowed to leave the Emirates Stadium this month before Bellerin's setback.