Moyes says Fernandes is United's talisman.
He said, "I think with Manchester United, Bruno Fernandes has given them another dimension completely. They had really talented players prior to that, but he orchestrates most of their good play, and at the end of it they've got great speed and ability in Greenwood, Rashford, Martial and Cavani now.
"They have really talented players, but we've got talented players as well, so we need to go there and put on a good show.
"We drew at Old Trafford at the end of last season and I thought for long periods of our game at home [in December] we certainly played as well as Manchester United.
"I'm really pleased with how the players have done and how they've come on. I think we gave Manchester United a very good game at London Stadium. It was a strange decision that changed the game when we were winning 1-0, and it got them back in it.
"Overall, I think we've competed quite well against the top teams. We've got a bit to go yet to be at that level, and that can take a long time.
"I hope we can go to Old Trafford and give them a very good game. We want to try and get through to the next round of the FA Cup."