The hosts have suffered five defeats from their last eight league games, while United recently improved to beat Stoke City. However, Riether said that his side's form could easily change.
"We know that it hasn't been a good start, we can't get away from that," Riether told the club's official website. "It's been a very frustrating period for us, but football is generally a game of ups and downs.
"I know the fans are frustrated, and us players share the disappointment of losing matches. When we lose a game, it stays with us for a long time.
"Manchester United are the reigning champions, but in recent years we have a good record against them at the Cottage. Last year we ran them close, with Wayne Rooney's late goal the difference on the day. It would be nice to be part of another memorable victory, and I believe that we have a chance."
Fulham are 14th in the Premier League, while United are eighth.