In my experience, the majority of 5*+ players I have acquired, have started out getting low ratings. Last season, I bought a 6* striker, and it took him 8 matches to get a rating of 7 or better, and 13 games before he scored. Since then, he's improved no end, to the extent that he finished the season with 6 m.o.m awards. I think they just have to settle in.