Celtic manager Brendan Rodgers confirmed that substitute Arne Engels was struck by a coin in the final moments of Thursday's Old Firm derby.
Rangers claimed a 3-0 victory -- their first league win in the derby since May 2023 -- but the game was marred by a temporary cessation to proceedings before the final whistle when Engels went down after an object appeared to be thrown from the home crowd.
Celtic defender Alistair Johnston said he had spoken to Engels and called for fans to show more respect.
"I saw that he was hit above the eye," Johnston said. "I'm glad it didn't catch him in the eye ... you just hope that everyone respects the players on the pitch."
"If that hits him in the eye, that's probably a really serious injury. There are humans out there. People need to be smart and not chuck things at people's heads."
Ibrox Stadium was filled with 51,000 home fans but no away supporters on Thursday -- the latest Old Firm derby to feature just one set of fans, although that is set to change later this season after a new agreement between the two clubs.