As throngs of home fans gathered outside the ground, which will close following the game ahead of a summer move to the Olympic Stadium, United's coach could not get through after already arriving later than anticipated.
When it did arrive, a number of items were thrown from the group of West Ham supporters including bottles, and it was announced kick-off would be shifted from 7:45pm to 8:30pm.
With post-match celebrations expected to last for up to 45 minutes, the delay could mean fans do not start flooding back onto Green Street until 11:00pm.
United captain Wayne Rooney confirmed the coach had come under attack as it struggled to get into the stadium.
"The coach got smashed up but we'll just have to go and prepare and get on with our job," he told Sky Sports
"I'm sure West Ham will be disappointed with their fans
"We know what we have to do, starting with tonight
West Ham are a good team."
West Ham co-chairman David Sullivan said both police and match officials had been "kind" to Manchester United by delaying the kick-off.
Sullivan seemed to blame Manchester United for not having arrived earlier, rather than point the finger at Hammers fans.
"Man United should have got here at 4pm," he said on Sky Sports
"They know it's our last game
They had the same problem at Tottenham a few weeks ago
I think the police and officials have been kind
I'd make them kick off at 7.45pm if I was in charge."
He continued: "I'm gravely disappointed, more for our supporters
What people forget is they've got trains and buses to get home and we've got a wonderful, wonderful show after the game for them
Many just can't stay any more."
Images and videos began circulating on social media, with the tinted windows of the United coach being cracked and damaged by missiles launched from the masses.
United boss Louis van Gaal said it was "not so nice" as his side prepared for a second delayed kick-off in the capital this year after their clash at Tottenham was put back following traffic problems.
"What do I have to tell? The images tell everything," he told Sky Sports.
"I have seen also the police were taping
It is also a lot of emotions for people but for us it is the second time in London we have to start later
It is better than last time because we have received our preparation time.
"It is not the first time I have seen that
It is not so nice
The way we have been received is not the proper way, of course
That makes all the influence on the players and that is a pity."
Source : PA
Source: PA