Trouble flared at half-time of Thursday night's match, with fans in the away section clashing with police.
At least two supporters were removed from the section by officers, although Police Scotland have not confirmed any arrests.
• READ MORE - 'The most disgraceful, annoying, and out of touch fans I have ever seen' - Feyenoord fan slams Rangers supporters after Europa League clashA spokeswoman for Police Scotland told the PA news agency: "An investigation is under way after five incidents of police assault within the west stand of Ibrox stadium during the Rangers v Feyenoord match on September 19.
"Anyone with information or images of the incidents is asked to contact Police Scotland."
Earlier in the day, a 22-year-old supporter was arrested in George Square, Glasgow, after a flare was set off.
• READ MORE - Feyenoord fans - including one wearing Celtic shirt - clash with police at Ibrox during Europa League match with RangersPolice escorted hundreds of Feyenoord fans as they marched to Ibrox ahead of kick off.
Rangers won the match against the Dutch side 1-0.
A UEFA spokesman said no action has been opened regarding the incident.