Michael Carrick cut a frustrated figure after Manchester United let slip the chance to usurp rivals Manchester City in the top-four.
Louis van Gaal's men arrived at Upton Park looking to be the party-poopers, knowing victory in the last-ever match at the Boleyn Ground would put the battle for Champions League qualification in their own hands heading into the season finale.
Anthony Martial's brace after a deflected Diafra Sakho opener but United on course for a crucial three points, only for Michail Antonio and Winston Reid to see West Ham sign off with an unforgettable 3-2 win.
The loss leaves the Red Devils two points of City heading in the final match of the campaign. United need to beat Bournemouth and hope their neighbours lose at Swansea as their far superior goal difference means a draw at the Liberty Stadium would be enough for Manuel Pellegrini's men.
"We have to (stay focused), that's the situation that has presented itself now," midfielder Carrick said.
"We were in control of it going into the West Ham game but we've let it slip. We have to win our last league game now and then see what happens.
"It's no good for us. We got ourselves back in the game and then didn't really help ourselves with giving away free-kicks. We have to be cleverer than that.
"We put ourselves under pressure by letting them put the ball in the box. In the end that's won them the game and we're very disappointed."
It was a frustrating return to Upton Park for former West Ham midfielder Carrick, who was keen not to blame the pre-match trouble outside the ground for their loss.
"Whatever has gone on beforehand, you've just got to get on with it," he said.
"We were here, we were ready and we had over an hour to prepare for the game. It's no excuse."
Meanwhile, Luke Shaw will not play again for United this season but is planning an early return to pre-season training in order to hit the ground running next term.
An exceptional start to the England international's second season at Old Trafford was cruelly halted by a horrific injury during September's Champions League clash at PSV Eindhoven.
Shaw suffered a double leg break in a tackle by Hector Moreno and there had been hopes the 20-year-old would feature for United before the season was out.
However, it is understood United are reluctant to rush him back to the first team, meaning he will have to wait until the 2016-17 to make his competitive return.
The severity of the injury saw Shaw undergo an operation in Holland, before undergoing minor surgery after returning to England.
The defender was, surprisingly, able to put weight back on his leg after just 10 days, with rehabilitation involving two hours in the pool each day once the wound had healed externally, as well as cycling and gym work.
Shaw underwent another minor operation three weeks ago to remove a screw, but things are understood to have gone without a hitch and the full-back has been training with United's Under-21s since last Tuesday.
The England left-back is still not involved in contact sessions, though, and will not feature in United's Premier League finale at home to Bournemouth or the FA Cup final against Crystal Palace.
That extinguishes any faint hopes of a place in Roy Hodgson's squad heading to Euro 2016, with Shaw now understood to be focusing on next season and planning to return to training in early June in a bid to be at his best when pre-season begins.