Brighton manager Graham Potter has confirmed Jakub Moder suffered an ACL injury in the draw with Norwich City on Saturday.
The Polish midfielder could now potentially miss the World Cup in November.
Potter told The Argus: "This is a devastating blow for Jakub.
"He has made such good progress for us this season and had just played an integral part in helping Poland qualify for the World Cup with victory over Sweden.
"Everyone will want to know how long he will be sidelined.
"It's too early to speculate other than to say he faces a significant period of rehabilitation.
"Naturally he will have all the support from the club during his recovery."
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