Arsenal defender Rob Holding will miss six to nine months after he ruptured the anterior cruciate ligament in his left knee against Manchester United.
Holding had to be carried off on a stretcher during Wednesday's 2-2 draw at Old Trafford, and the club said on Friday that he will need surgery to repair the ligament damage.
The estimated timeline means Holding could be back to full fitness during preseason ahead of the 2019-20 campaign, although a nine-month recovery would mean he misses the start of that season as well.
The injury is a big blow for the 23-year-old who had established himself as a regular in recent weeks and was enjoying the best spell of his Arsenal career.
The centre-back has started the last nine Premier League games in a row and all five of the club's Europa League matches, his most consistent spell since joining from Bolton in 2016.
He is the second Arsenal player to suffer a long-term injury this season, with Danny Welbeck recovering from surgery on his right ankle.
Arsenal coach Unai Emery is set to receive an instant reinforcement, however, as captain Laurent Koscielny is now fit again after recovering from a torn Achilles tendon.
Emery said on Thursday that Koscielny is likely to make his first appearance of the season against Qarabag in the Europa League on Thursday.