Why is my player's strength changing?

the current morale (MO) and form (FO) of a player can directly influence his current strength. The player receives a strength penalty if morale or form is very low. The player receives a strength bonus if the form or morale is very high.You can also improve the form and morale of players with the help of training sessions and training camps.Players suffering 3 severe injury will drastically lose in strength. Consider selling off such players before that happens.