Aston Villa have confirmed defender Diego Carlos has ruptured his Achilles tendon and will require surgery.
The centre-back picked up the injury in Villa's 2-1 win over Everton on Saturday lunchtime, in a blow to Steven Gerrard's season.
The Villa boss said he was "extremely worried" after Carlos went off and refused to rule out signing a replacement before the window shuts.
Carlos, who was signed for £26m from Sevilla earlier this summer, will begin a rehabilitation programme for his injury once he has had his operation.
The Brazilian has started both of Villa's games this season, partnering Ezri Konsa in the defeat to Bournemouth and Tyrone Mings in Saturday's win over Everton.
Konsa, Mings and Callum Chambers make up the rest of Gerrard's options at centre-back, with Kourtney Hause out due to a knee injury.
Gerrard 'extremely worried' by Carlos injury
Aston Villa manager Steven Gerrard after the win over Everton:
"While the window is still open, you can never rule it out [signing a replacement].
"We will have to wait and see what the outcome is of Diego Carlos' injury and why he's gone off the pitch because I am extremely worried about that situation.
"We are going to send him for an MRI scan, but it doesn't look good.
"That might tarnish how I'm feeling from a personal point of view, I'm extremely happy with the win and the performance, but to lose someone that is so important to us, that could tarnish the win."