Stamina is absolutely vital for any players. Many managers consider anynithing below solid stamina as a waste of resources in a league or cup game.
If a player does not have enough stamina, a serious drop in his performance will be noticed, particularly during the second half of matches. You will notice his drop in performance in his stars rating. The total number of stars shows his average performance during the game, while the number of yellow stars shows his performance at the end of the game. So the more grey stars, the more performance is lost due to low stamina.
You can decide shich percentage of your normal training to dedicate to stamina (minimum 5%), the amount needed to maintain a certain level of stamina depends on the age of the players. Stamina training efficiency will drop at higher intensity level, it´s better to train a little every week than a lot now and then.
Midfield Stamina Effect
The following percentage gives the impact of stamina on the midfield rating for a player with the same form/experience/playmaking rating