The Gunners are favourites for the clash at the weekend as they face a Royals side that find themselves firmly in the relegation mire, sitting seven points adrift of safety.
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"It is very difficult [against teams near the bottom of the table]," he told the club's official website.
"They are fighting against relegation, and we are fighting to be in the top four.
"We may be favourites, but we know it's not going to be an easy game. But if we can play our game, we know we can score goals."
Reading provided Arsene Wenger's men with a real scare when the two sides met in their 7-5 Capital One Cup clash earlier this season, and Diaby insists that the Gunners need to show the same fight they did when overturning a four-goal deficit to progress.
"It was crazy," he added. "The way we came back and responded after half-time was unbelievable.
"At 4-0 down, you know it'll be really hard but we showed really good character and that is what we need to carry all the time, especially now."