With no fit recognised centre-back and attacking players such as Ricardo Vaz Te, Stewart Downing and Andy Carroll all missing West Ham are down to the bare bones as they look to escape from the Barclays Premier League relegation zone.
A trip to Craven Cottage on Wednesday follows an enthralling 3-3 draw at home to West Brom at the weekend and, with a treatment room full of first-team regulars, McDonald believes another stalemate would suit the Hammers.
"Not to lose the game is important, that's for sure," McDonald, who was on media duties as manager Sam Allardyce was busy getting ready for January transfer window activity, said when asked if the Fulham clash was a must-win game.
"When you're playing a team in and around you that is the thing to do.
We've scored three goals against West Brom but conceded three, we need to stop conceding and keep scoring. We are creating chances but it is difficult when you haven't got your defenders playing.
"In this scenario, a win is great and puts you right up to the table but it's important not to lose.
"If we had beat West Brom it would have pushed us up the table and then a win at Fulham would have moved us towards halfway.
"That is how very, very close it is and how tight it is and how important it is to pick up points and respect the point if you get it, it could take you further up the league."
With club-record signing Carroll missing since the start of the season it has been in attack where Allardyce's side have struggled most this season.
But, with James Collins, Winston Reid and James Tomkins all out, it is more options in defence which McDonald believes is the manager's immediate priority.
"It looks as though they [Collins, Reid and Tomkins] are not going to be fit for the Fulham game, we will assess them ahead of the Forest game but it is more likely they will be involved in the Cardiff game," he said.
"We have a few defensive problems and with the window coming up Sam is working non stop.
"We need some reinforcements to boost the players we have here so he is working behind the scenes with everyone else. His phone is going non stop.
"We have three main centre halves injured at the moment. We are aiming to improve and the two main places are in defence and in attack."
Modibo Maiga came off the bench to score in the draw with West Brom but it was the Mali international's first Premier League goal in over a year.
Carlton Cole's short-term deal is also drawing to a close and, with Mladen Petric almost certain to leave the club in the coming days, the news that Carroll has started training with his team-mates will be a huge boost to Allardyce.
Having spent last season on loan at Upton Park, the England international moved on a permanent deal during the summer but has been kept on the sidelines with a heel complaint.
McDonald confirmed the 24-year-old is working his way back but that it will take time until Carroll can be welcomed into the first-team fold.
"Andy did a training session yesterday which is very encouraging," added McDonald.
"It's early stages but is a work in progress with him; work one day on the training pitch, massage and rehab the next day and let's see how it goes. It's a positive he is in training but it is a slow process.
"When you are out for seven months I don't think you can rush anybody back. Obviously, we are all desperate for him to come back and get into the team as quickly as possible but we have to be sensible and he is getting closer."
Allardyce could again be forced to play four full-backs in his denfence at Fulham while he also has the option of deploying Alou Diarra at centre-back.
Either way, only a win would give West Ham any chance of getting out of the bottom three - whilst the point gained from a draw could only lift the Hammers as high as 18th.