“We await the match delegate’s report and we would urge any fans with relevant information to contact Police Scotland.”

Rangers had condemned “in the strongest possible terms” the incident in which substitute Engels 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 said they believe the police are investigating two separate incidents.

“The repeated targeting of our players and staff with missiles is quite appalling and completely unacceptable,” they said.

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 Premiership to 11 points and could leave Rangers facing punishment.

Celtic right-back 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 Canadian added.