The Hammers are at Old Trafford for the FA Cup third round replay and Diarra is targeting a surprise victory against the Premier League leaders.
"It's not going to be easy in Manchester," Diarra told the club's official website.
"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.
"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."