Running around doesn't necessarily mean you're good defensively. Fred is no Kanté. He has bad positioning, frequently miscontrols the ball and loses possession often. Is that what you call someone defensively sound?
His best work happens between the attacking third and the middle. That being said, he's not a clever reader of the game and does not anticipate what the opponent is about to do. He's a chaser and a nuisance to the opponent, but he's extremely easy to play around and dribble through if the opponent has a defence/midfield comfortable on the ball. He is also quite weak and loses most of his 50/50 duels.
I don't get this, really. Our best performances and midfield trio came from playing Casemiro, Eriksen and Bruno. If we then buy Mount who is an upgrade on Eriksen, particularly in terms of work rate, strength, tenacity, defensively and pace, how exactly can you come to the conclusion we're going to do worse and get cut through easily?