West Ham co-Chairman David Sullivan played down the incident, telling BBC Radio 5 live that their opponents should have arrived earlier.
He said: "It is depressing really, it will be a late night for our fans and some of them won't be able to stay.
"I don't understand why United couldn't get here at 4pm. They could have got here early. They knew it would be busy. It's crazy.
"There was congestion in the street and they couldn't get the coach in. There were people around the coach, but there was no attack on the coach.
"If you check the coach there won't be any damage to it. If we arrived late at Old Trafford they wouldn't put the kick-off back."