Veteran midfielder Ryan Giggs is the latest United casualty and although he is set to be fit for the weekend and defender Rio Ferdinand is also close to a return, the Premier League leaders are depleted for the last-16 first-leg trip to France.
"You are working through a really difficult month in December in terms of the number of games, and then through January we have had a few games too, and it's just the strain of it all," Ferguson told a news conference on Tuesday.
"But you know, we have got a strong squad with us and we carry on.
"I think they are a very powerful team, very athletic, strong defenders. They, like us, have a few injuries."
Ferguson is also wary of the raucous reception inside the Stade Velodrome, where fans create the biggest roar in French football.
"We know from being here before, and also having been to many games here, that it's a fantastic atmosphere here, and that helps Marseille," he said.
Marseille boss Didier Deschamps waxed lyrical about the Premier League leaders.
"They have won lots of games and lots of titles nationally and internationally, and evidently this is symbolised by their manager Ferguson, who is on his 24th season as manager, which is something very exceptional," he said.
"There's this culture of winning, which you find in big clubs, they can change players and can lose good players, but they're replaced by others, and there's always a message passed on by the older players and the guarantee is the manager."
