Pereira is set to be out for six months with a ruptured achilles.
The defender was withdrawn from the 1-0 pre-season friendly win over Sevilla at the King Power Stadium on Sunday and has undergone an operation on his achilles this week.
"Sadly, Ricardo Pereira will be out up to six months," the Leicester boss said.
"He ruptured his achilles, which is just such a shame because a lot of our game idea over the summer was based around full-backs and him in particular. He was looking so good. It's a massive blow to him. He had his operation last night and he'll recover now, get strong again and come back better I'm sure.
"He'll be a big miss for us but we have to push on."
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