Only having one player with one certain number of stars afect there performance.
To explain better.
Last season i bought a 20years old fast trainer 1 level to gaine is 6 star. In the next 5 matchs until he got the star he scored 13 goals and had always at least 8. After getting the 6star, until the end of season only scored more 6 goals, and the ratings got lower. He was my only scout.
Probably is like having Ronaldo or Messi in a second division Team, they can not make the diference.