The Italian was rubbing his eyes when he came out late for the second half in Thursday's defeat in Ukraine and was substituted soon after.
"Mario had an allergy - I don't know what. His face was swollen - I don't know why," coach Roberto Mancini told reporters.
"He wanted to play in the second half but he had this problem 10 seconds before coming out."
British media reported the striker was sensitive to a certain type of grass.
City face Championship Reading at home in the FA Cup Quarter-Finals on Sunday.
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