"He will be out of the squad for between three and six weeks, then we shall see if he can recover quick," Puel told reporters on Thursday.
"It is a sprain, not a break... it isn't serious, he needs rest and rehabilitation," the Frenchman added.
Gray, 22, has played in all Leicester's six league games this season and was called up to England's squad for the friendly against Switzerland this month but did not make his debut.
Leicester are ninth in the league with nine points after six games and Newcastle are 18th and still seeking their first win.
(Reporting by Shrivathsa Sridhar in Bengaluru, editing by Ed Osmond)
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