South Wales Police have appealed for witnesses after a flare was set off during Sunday's draw between Swansea City and Manchester United at the Liberty Stadium.
The flare is believed to have been thrown from the away end following United's opening goal on 15 minutes.
The object hit the roof of a stand before falling back down into the crowd below.
"This was a very foolish act by an irresponsible individual," said Detective Inspector Jeff Handley. "We are fortunate that nobody was seriously hurt today.
"It's important that anybody with information contacts us so that we can identify the individual and let him or her explain their actions to a court."
Witnesses with any information have been urged to call South Wales Police on 01656 655555.
