The midfielder was forced off during the FA Cp win against Tottenham last week.
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"We are checking him and he is visiting some doctors and we are going to know about him in a few days, but of course he is not available and maybe for a long time," said Emery.
"Hopefully it will not be such a long time but we are checking him."
On the decision to sell Donyell Malen to Roma this week, Emery added: "It wasn’t an easy decision but it makes sense. Players always have the chance to get a deal to leave the club. He opened his door to listen in case some team can give him something better than us."