I don't know why people are worried and thinking that after the world cup game players will change in their performance. . I believe whether world cup or no world cup changes in performance can occur in a team at anytime, I don't think world cup game is a good excuse for teams to perform poor. World Cup game is an opportunity to build up the skills in players. Manchester United are not doing bad in performance, they still have good players but they need to improve more.
It could be that a player's decline in performance is caused by things other than their participation during the World Cup, this could mean that the player is not controlling his diet or physical fitness during the holidays. The fact that not all the players will be involved in the World Cup will allow them to be a bit less in control.
The question is, have you ever heard that a player is no longer at his best after a break or vacation due to weight gain? This will probably answer your questions and doubts.
Decreased player performance can also be caused by injury, there are several cases after a player is injured and cannot return to his best performance. Players who don't participate in the world cup can still control, they can do regular exercises during breaks to maintain their physical fitness.
Not all players who play in big teams like Manchester City, Arsenal, Liverpool and Chelsea are called up by the national team. Those who are excluded from the national team can still do something that can maintain their performance. During the break they will continue to do training with other players, the coach has a responsibility to keep his players disciplined and carry out routine activities with the club.
Professional players will always maintain a special diet and exercise during the holidays to maintain their weight, they will do anything to keep their performance always in the best condition.
Very rarely is there a case like your question, they will not do something that could destroy their career.
Concerns about the decline in player performance after the break are unfounded, i never thought it would happen. All players will continue to do things that can improve their performance during the world cup break.