Florian Lejeune has suffered a cruciate ligament injury that means the Newcastle United defender will need surgery.
Lejeune was absent for Saturday's pre-season friendly away to Porto and is set for an extended spell on the sidelines.
The centre-back confirmed the news in a social media post, vowing to return to action as soon as he can.
"During my last training session, I left the field due to an injury," Lejeune wrote on Twitter on Sunday.
"After further medical examination, the doctor told me I have a cruciate ligament injury. I'll have surgery in the next few days.
Search for replacement
Lejeune's injury is a major blow to Newcastle given its relative lack of squad depth, although it did complete the signing of defender Fabian Schar from Deportivo La Coruna this week.
It has also signed Japan international striker Yoshinori Muto from Mainz, subject to a work permit.
Manager Rafael Benitez wants further arrivals before the August 9 deadline, though, with Newcastle set to raise funds from the imminent sale of striker Aleksandar Mitrovic to Fulham.
"The reality is we have lost Lejeune – he has a cruciate ligament injury, so we will lose him for a while. He will need an operation, and he will need some time," Benitez said after Newcastle drew with Porto.
"We needed a centre-back before [Lejeune's injury] and still we need maybe another one. It's bad news about Lejeune but to see Fabian Schar doing well is important; he will give us this extra option we didn't have.
"The deal is done with Fulham and [Mitrovic] has an agreement. So, hopefully we can bring someone to replace him, quickly.
"It's something we were expecting and it's now a case of looking to see if we can bring someone else to replace him.
"We are working and trying to bring in new players to bring in new competition. It means we can bring in another striker and we can be stronger.
"We did this business with Muto because we needed to do something more. I don't control the accounts but, if you do the maths, we have enough money."
"I'll do everything I can to get back as quickly as possible on St James' Park."
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