Stamina is absolutely vital for any players who use playmaking. For inner midfielders it's not a secondary skill, but a primary one together with playmaking. One of the most common mistakes by playmaking trainers is to slowly increase stamina for their players. Playmakers with anything below solid stamina are a waste of resources in a league or cup game.
If a player does not have enough stamina, a serious drop in their playmaking contribution will be noticed, particularly during the second half of matches.
From August 2006 new rules apply. The stamina will be reduced over time, with approximately 1-2 levels lost per season.
From July 2007 other rules are applied, namely that the stamina training is a percentage of your normal training, varying between 5% and 100%.
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