An abject 2-0 defeat at arch rivals Liverpool in the first leg of their Europa League tie on Thursday had the critics again sharpening their knives and tongues.
United greats Paul Scholes and Rio Ferdinand were damning in their assessment of another lifeless display from Louis van Gaal's side.
The loss at Anfield has made it odds-against United reaching the quarter-finals of the competition in next Thursday's home leg and keeping alive a potential route back into the Champions League.
Qualifying via a top-four finish in the Premier League also looks beyond United, who are sixth in the table, in their current form.
Winning the FA Cup would only guarantee entry into next season's Europa League but delivering a piece of silverware looks increasingly like van Gaal's only hope of staying as manager at Old Trafford into next season.
Even that might not be enough to save the Dutchman's job following a season of apparent under-achievement.
But with United failing to win anything since the last of Alex Ferguson's Premier League title successes in 2013, and having enjoyed the last of their 11 FA Cup successes way back in 2004, going on to lift the trophy would provide some solace and salvation to a troubled campaign.
Defeat to West Ham, who have won their last four games and lost only one of their last 10, would not only snuff out that hope but also increase the pressure on van Gaal, whose position has been under the closest scrutiny ahead of each of United's previous three wins in the competition this season - against Sheffield United, Derby and Shrewsbury.
United will have to try and avert another crisis without Juan Mata, who captained the side against Liverpool but who serves a one-match ban following the first red card of his career in last weekend's 1-0 Premier League defeat at West Bromwich Albion.
But the Spaniard is urging his team-mates to respond positively to their defeat at Liverpool and keep open at least one avenue to a successful end to the season.
“The FA Cup is very special for all of us and it is against a very good team like West Ham,” Mata said.
“We need to recover as soon as possible and then focus just on the FA Cup. Then, after the weekend, we can look at the Europa League again.
“It was not a good performance and not a good result obviously. But we need to move on, we need to win on Sunday. It is a chance to play at Wembley again.”
Winger Jesse Lingard will be back in United's squad after missing the loss at Liverpool through suspension but captain Wayne Rooney, winger Ashley Young and defenders Phil Jones and Antonio Valencia are still to return.
Meanwhile, West Ham manager Slaven Bilic is treating United's latest slip with a degree of caution.
“We all know that their game against Liverpool last night was not a normal game,” he said.
“It was at Anfield in an intimidating atmosphere and it is just a one-off game.
“On Sunday, they will be playing in front of their own fans and so unfortunately for us, we expect a better performance than last night (Thursday's game).
“It is Manchester United and although they are under pressure they are capable of playing better and they have always bounced back after a tough loss, so it will be a tough game.”
The Hammers' recent form, however, means Bilic's side will travel to Old Trafford believing they are capable of reaching the last four.
“We are confident, of course,” he added. “Not because Manchester United didn't play well in the game against Liverpool, but because we are playing well.” – AFP
Original source: Tricky test for troubled United