Florian Lejeune has suffered a cruciate ligament injury that means the Newcastle United defender will need surgery, leaving the club in search of replacements.
Lejeune was absent for Saturday's pre-season friendly away to Porto and is set for an extended spell on the sidelines.
The 27-year-old's injury is a major blow to Newcastle given their relative lack of squad depth, although they did complete the signing of defender Fabian Schar from Deportivo La Coruna this week.
They have also signed Japan international striker Yoshinori Muto from Mainz, subject to a work permit.
Manager Rafael Benítez confirmed that his club are on the verge of selling forward Aleksandar Mitrovic to Fulham and the manager hopes that they can quickly turn those funds into cover for Lejeune's injury.
"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."
Newcastle open their Premier League campaign with a clash against Tottenham on August 11.