Leeds United manager Daniel Farke says centre-back Joe Rodon will be out for between eight and 10 weeks with a “serious hamstring injury”.
The Wales international, 28, was forced to come off after 34 minutes in his side’s 1-1 Premier League draw against Brentford last Sunday.
Rodon had played in the season-opening league win at Nottingham Forest and a Carabao Cup success against the same opposition.
“Bad news, serious hamstring injury,” said Farke, whose side travel to Brighton on Saturday (15:00 BST).
“He will be out for eight to 10 weeks. Definitely not playing before the next international break.
“A big blow for us because he’s a key player. Puts more pressure on us to get a wing-back – we may need James Justin at centre-back.
“[It is] important Joe comes back as quick as possible. Need a bit of luck, but we also have the January window to react if necessary.”
Leeds signed French defender Melvin Bard on deadline day to add to their left-back options, where the versatile Justin has been covering too for Sweden international Gabriel Gudmundsson, who picked up a hamstring injury in pre-season.
Rodon’s absence will also be a setback for Wales, who face Portugal on 24 September and Denmark on 27 September in away Uefa Nations League Group 4 matches during the next international break.
They also host Norway and Denmark in home Nations League games at the start of October.