The Slovenia international was playing for the Magpies' Under-21s on Monday night at Aston Villa when he picked up the injury after a challenge from Samir Carruthers.
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Pardew told The Shields Gazette: “I think Haris knew straight away it was a serious injury – it was an emotional night.
“He’s had so many injuries, it was perhaps one too far for his own mind. It was a very emotional night for us all – we’re upset for him.”
Midfielder Dan Gosling was stretchered off in the same match on Monday and the club confirmed he will be out for up to a month with ankle ligament damage.
The Magpies’ next Premier League fixture comes after the international break in the form of a crunch derby clash at Sunderland on October 21.