The England international was injured in a challenge from Adam Le Fondre in the opening minutes of Everton's 3-1 win over Reading at Goodison Park.
He had to be immediately replaced and Moyes said: "I genuinely haven't seen it again but I thought on the pitch it didn't look good.
"Jagielka knew about it and it is a bad one. He will have surgery to stitch up his ankle.
"It is a bad one because he is a key player for us."
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