The West Ham boss was unable to bring in any defenders on loan in the closing days of the winter transfer window and injuries have left him with very few options when Louis van Gaal's side arrive at Upton Park.
Winston Reid and James Collins are both absentees and Allardyce is likely to ask fit-again Senegalese midfielder Cheikhou Kouyate to step into the defence and partner James Tomkins, himself only just back from injury.
With club-record signing Andy Carroll struggling for fitness after suffering an ankle problem at Liverpool last week, the Hammers are light on numbers as they go in search of just their second Premier League win of 2015.
"(The injury list) is particularly worrying at the moment," said Allardyce.
"I've had to bring a list of the injuries with me
James Collins, Winston Reid, Guy Demel, Diego Poyet, Doneil Henry and Andy Carroll lie in our medical room at the moment.
"We're not sure how many of those will make it for Sunday
With such a big week coming, three games in a week, there'll be very few risks taken, unless they're 100 per cent fit
If we play them it may exacerbate the injuries so we will go to battle with what we've got.
"It is why we went all out for a defender in the window after the Liverpool game
However it didn't work out, we were only in a position to get someone in on a loan but we couldn't pull it off.
"Everyone that wanted to come wanted a permanent deal, which we couldn't do at that time, so that's the position we're in
Someone will have to fill in at the back and maybe stay there for a while."
Transfer deadline day was quiet for most Premier League sides, including West Ham, but it was not through choice for Allardyce's side.
Jack Sullivan, the son of co-owner David Sullivan, tweeted that the club had seen as many as eight deals break down across the course of Monday - with Tottenham striker Emmanuel Adebayor's potential last-minute loan deal falling through as Spurs did not want the Togo international to join a direct rival.
"A lot of moves were blocked for various different reasons, so I'm not that bothered," Allardyce said of the lack of business in east London.
"From our point of view it left us a bit frustrated, and especially Carlton Cole on the basis that he would've moved had Adebayor joined, as it happened we couldn't afford to let him go.
"It was a frustrating day, originally it didn't look like we were going to be active until the very end, but then we picked up five injuries and now we are very short
We don't have a problem with (Tottenham), it is their decision
If anything that was the only loan available, it just didn't work out at the end."
Cole will now be in the squad for the game against United after his proposed move to West Brom was cancelled late on
The striker could yet lead the line for Allardyce's side with Carroll sidelined and Diafra Sakho's position in the side still in doubt following the fine handed down by FIFA for West Ham's breaching of the regulations
Sakho came off the bench to score in the 1-0 FA Cup win over Bristol City despite the fact he had not travelled to link up with Senegal at the African Nations Cup due to a back injury.
West Ham, who maintain they did not break any rules, blocked any questions on Sakho's situation at Allardyce's pre-match press conference and the former Bolton boss was bullish when asked if his top goalscorer could play against United, answering simply: "We will see."
In better news for the club, a record shirt sponsorship deal has been announced with global betting brand Betway.
The company have struck what is understood to be a three-and-a-half year, £20million deal.
The move comes after previous sponsors Alpari entered insolvency last month.
Source : PA
Source: PA