The two sides face off at Old Trafford in their FA Cup third-round replay clash following the dramatic 2-2 draw at Upton Park, where a late Robin van Persie equaliser saved the Red Devils from crashing out of the competition.
"It's not going to be easy in Manchester, they're in confident mood, even if they make a few changes. They have strength in depth and a lot of players and can rotate," Diarra told the club's official website.
"For us, it's a question of raising our game in comparison to our performance at Sunderland and start the match just as we did in the home game.
"It's not an easy game but in the cup anything is possible. We'll have chances at Old Trafford."
Diarra joined the Hammmers from Marseille last summer but due to a number of injury problems he has managed to play only four games so far this season.