Manchester United manager Louis van Gaal put on a brave face and refused to blame an attack on his team's bus for the serious setback to his side's Champions League hopes at West Ham United.

Tuesday's gripping 3-2 loss, which was preceded by the United team bus being pelted with cans and bottles outside the east London ground, leaves Van Gaal's men needing to beat Bournemouth in their final match on Sunday and hope Manchester City lose at Swansea City if they are to creep into the Premier League's top four.

"No, it is still not gone," Van Gaal said of a top-four finish following the final match of West Ham's 112-year tenure at the Boleyn Ground.

"In the last match of the season we can pass over Manchester City. They can lose and we can win. Then we are fourth. You can count, I think?"

Van Gaal said to blame the bus attack for the result would have been an "excuse", but he conceded that it could have affected his younger players.

"I have a long experience in football and there are players who don't have the experience," said the 64-year-old Dutchman.

While United still have an FA Cup final date with Crystal Palace to come on May 21, missing out on a top-four place would represent a massive disappointment, but Van Gaal believed Swansea could give City a hard battle.

"I don't think it is not likely. In this league it happens a lot and Swansea City has a very good run.

"It is a mental blow for the players, but against Bournemouth we have to give everything so that we don't give Manchester City a free trip into the Champions League."

The Football Association announced an investigation after United's bus was pelted with missiles outside the ground, causing a 45-minute delay to the kick-off.

The Metropolitan police said one officer and one member of the public had sustained minor injuries.

United goalkeeper David de Gea also appeared to have objects thrown at him during the game.

West Ham co-chairman David Sullivan blamed the visitors for not arriving earlier, but United captain Wayne Rooney said that "West Ham as a club will be disappointed with what the fans have done".

A video posted on United midfielder Jesse Lingard's Snapchat account showed several United players sheltering on the floor inside the bus while Lingard mockingly shouted: "Mummy!"

Needing to win to keep their top-four destiny in their own hands, United fell behind in the 10th minute when Manuel Lanzini teed up Diafra Sakho to score.

Anthony Martial scored twice to put United ahead, but Michail Antonio equalised with a bullet header before Dimitri Payet teed up Winston Reid to head home an 80th-minute winner.

The goal was a fitting conclusion to West Ham's 2,398th and final match at the Boleyn Ground -- also known as Upton Park -- which the club will leave next season for London's Olympic Stadium.

The match was followed by an on-pitch ceremony featuring pyrotechnics and London taxi cabs, while club greats Trevor Brooking, Martin Peters and Paolo Di Canio were presented to the crowd.

"It was a special game," West Ham manager Slaven Bilic told Sky Sports.

"It was big, big pressure for us, although I told the guys, 'However we finish the last couple of games, this season has been great for West Ham in terms of breaking a few records and everything.'

"We were till the end in a couple of competitions and battling for the Europa League and a good run in the FA Cup, so it was good, very good or excellent, but we were under pressure.

"We didn't want to lose our last game at this fantastic stadium. We just wanted to win and this was a win of character."

West Ham returned to sixth place, reclaiming a Europa League berth from Southampton in the process, ahead of their final game of the season at Stoke City on Sunday.