Striker Peter Odemwingie and midfielder Jerome Thomas are both battling to be fit for the game with knee and calf knocks, respectively.
The duo have been responsible for seven of the nine goals that West Brom have plundered this season in the league, but Di Matteo is refusing to show any fear.
"They are both struggling," he conceded. "Thomas has a slightly better chance of being available for Saturday than Odemwingie.
"Friday is the last day of training before the game, so we will wait until then and then decide.
"Peter is a very important player for us, but we have to think about the rest of the season as well, it's not just Saturday.
"We are a team with some individually good players, and we know the importance of those players, but I think we try to win as a team.
"That's the only way forward for us. We will still try to be competitive.
"We go in with belief, and if the opposition has a better day or is stronger than us than we'll have to deal with that."
Manchester United's stuttering form in their last two outings - draws against Sunderland and Bolton - has given fresh hope of the Baggies claiming a positive result.
Di Matteo, though, has dismissed claims that it is a good time to face United.
"I've heard all that before," he added. "When we went to Stamford Bridge everyone said, 'It's a good time to play Chelsea, they haven't won a game in pre-season'. It ended up 6-0.
"For me they are a great team. There's never a good time to play teams of that quality. It's going to be a difficult afternoon for us."
The opening fixtures were particularly cruel to the Baggies and have seen them travel to Chelsea, Liverpool, Arsenal and now Manchester United in the first two months of the term.
Despite this, they are the highest-placed Midlands team in the league in sixth.
"We looked at our first four away games, and having to play all of those teams, it's tough," Di Matteo added.
"Having said that, I don't think that if you go to Stoke City or West Ham it's any easier, they just throw different challenges at you."