Rangers have condemned “in the strongest possible terms” an incident in which Celtic’s Arne Engels was struck by a coin during Thursday’s Old Firm game.
The substitute fell to the ground and needed treatment after being hit when going to take a corner towards the end of Rangers’ 3-0 win.
There were no Celtic fans inside Ibrox because of the ongoing dispute about away ticket allocations and Rangers said they will assist police with their investigation.
Celtic manager Brendan Rodgers confirmed that Engels was “OK” but added that “an inch lower, he would have been [hit] right in the eye”.
The incident came near the conclusion of a derby that cut Celtic’s lead at the top of the Scottish Premiership to 11 points and could leave Rangers facing punishment.
Celtic defender Alistair Johnston saw Engels being hit and cautioned “everyone to be smart” when at matches.
“It’s humans out there, someone’s son, so let’s refrain from chucking things at guys’ heads,” the defender added.
