The Juventus defender went down after a challenge early in the second half in Poznan and was replaced by his club teammate Bonucci
The Juventus center back was involved in a tangle with striker Kevin Doyle and was immediately replaced by his Bianconero teammate Leonardo Bonucci.
The Azzurri won the game 2-0 courtesy of goals from Antonio Cassano and Mario Balotelli and have progressed ahead of Croatia and alongside Spain, which has topped the group, to the quarterfinal stage, but the injury could be a major loss for Cesare Prandelli's team.
Chiellini had recovered from a thigh injury that he picked up in a win over Atalanta towards the end of the Serie A season, and there were doubts about whether the Old Lady defender would make a full recovery ahead of this summer's tournament.
The substitution was swift and possibly a precaution, but a reoccurrence of the thigh injury could see him miss the rest of the competition.
The 27-year-old has been central to Prandelli's defense in its draw against Spain, despite also making a mistake which led to Mario Mandzukic's goal in its match versus Croatia.
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