Greater Manchester Police (GMP) said "it became clear that many of those present were not intending to exercise their right to peaceful protest" and that "flares were let off and bottles thrown at officers".
They added that protestors getting into the ground meant "some United staff having to lock themselves in rooms".
"Those in the stadium were evicted by officers but outside on the forecourt, hostility grew with bottles and barriers being thrown at officers and horses," said GMP.
"Two officers have been injured with one officer being attacked with a bottle and sustaining a significant slash wound to his face requiring emergency hospital treatment."
GMP assistant chief constable Russ Jackson said: "The behaviour displayed by those at both Old Trafford and the Lowry Hotel was reckless and dangerous.
"We have launched an investigation and we will be working closely alongside partners to ensure we establish the full circumstances surrounding the events and prosecute those responsible."
BBC Sport has been told the disorder included serious clashes with police around the entrance to the Munich tunnel, which runs behind the Sir Bobby Charlton Stand at Old Trafford.
"Our fans are passionate about Manchester United, and we completely acknowledge the right to free expression and peaceful protest," said United.
"However, we regret the disruption to the team and actions which put other fans, staff, and the police in danger.
"We thank the police for their support and will assist them in any subsequent investigations."
Liverpool, who were also involved in the failed attempt to join the ESL, said they were in "full agreement" to postpone the game.
"It is our position that public safety must be the number one factor in any such decision, with the ability to provide a secure environment for the participants, staff and officials being a particular priority," said the Anfield club.
"It was clearly not possible for this to be guaranteed due to a situation which escalated rapidly."
https://www.bbc.co.uk/sport/football/56960091