Theoretically, it is impossible to improve the training quality by adding a coach whose star rating is not greater than the original coach, except for the following two factors:

When the number of coaches is insufficient, increasing the number of coaches reduces the workload of the original coaches.

Although the star rating of the new coach is lower than that of the original coach, as long as his potential is greater than or equal to that of the original coach, he can also achieve the effect of promotion.

Of course, the training quality of players is mainly determined by their potential, injuries and competitions.