How can you replace a player when you dont have the right replacement for the player? Manchester United substitute players cannot make any changes when they come into the field. For quite some time, Garnacho has been starting replacing Antony, when he is subbed for Antony there isn't any good performance from him as well. Manchester United should sell their players and buy new players or better they should bring young boys from the academy. Because it pains me a lot to see that it's their first team that is having this bad performance its better that the academy boys lose to some teams in the EPL than see the almighty Manchester United players including Rashford doing nothing.
This will be more problematic when the substitutes have skills below the core players so that the rotation carried out will have a worse impact. Although in some cases this actually happens better where the substitutes can play a much better role. I remember how Maino's debut was and who said he had the best ability as a debutant but he proved he deserved to be given trust so that his game got better and improved.
The problem faced by Man United is because their players are too selfish and they don't play collectively because they are too focused on individual achievements. Garnacho and some other players held the ball too much and sometimes they did not pass at the right time. Even though the chance of a goal would be much greater when the ball was passed to another player, that's what happened.