A woman received burns on her leg after Sheffield Wednesday supporters threw smoke canisters at Leicester City fans during the match at the King Power Stadium on Saturday.
Two men were arrested but were later released without any further action being taken. The woman received medical treatment at the ground for a slight burn to her leg.
A club spokesman said the smoke cannister that hurt the woman was one of two thrown by Wednesday fans into the City enclosure.
Dom Bray, 34, of Aylestone, Leicester, believes he was hit by the second.
"Two minutes before kick-off I felt something hit me in the back," he said. "When I turned around and saw the smoke I thought it might be a flare and could have melted my jacket or one of the seats.
"A few lads behind me came looking for it. They found it under the seat in front of me and threw it back – then it was chaos."
Dom reported the incident to club stewards at half-time.
"Fortunately, I wasn't hurt and didn't realise this lady had been until after the game," he said.
"It's bang out of order and makes me so angry – you don't go to a game expecting some missile to be thrown at you."
A Leicester City spokesman said: "Despite selective searching of visiting supporters upon entry, two smoke canisters were smuggled into the away end of the stadium and activated, resulting in one home supporter requiring examination for a minor injury to the lower leg.
"CCTV operators identified two suspects, who were arrested at the end of the game."
A Leicestershire Police spokesman said: "Two people were arrested in connection with this and have since been released with no further action."