Milan visit Barcelona on Tuesday for the second leg of their last 16 tie, holding a 2-0 lead from the first leg.
Separate Milan statements said Pazzini's problem had been identified and that he would not be travelling to the Nou Camp.
Pazzini, who joined the club from neighbours Inter Milan at the start of the season, partnered Stephan El Shaarawy in the first leg as Milan's top striker Mario Balotelli is ineligible having played for Manchester City in the group stage.
Pazzini scored in the first half of Milan's 2-0 win at Genoa on Friday but went off injured shortly afterwards.
Milan said he had also suffered heavy bruising although they did not say how long they expected him to be out of action.
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