Diarra, who joined from Marseille in the summer, endured an injury-plagued beginning to life at the Boleyn Ground.
"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 admitted to whufc.com.
"Danny Welbeck and Wayne Rooney are two very good players of international, world class. I had the opportunity to play against them at the Euros. That means I know them a little bit, I know their quality. They're two very good forwards, both very dangerous, and we'll have to keep an eye on them and not give them any space whatsoever."