Using the same players all over will tire them. They won't show it but their rate will drop significantly. My ace striker started at 7* and scored 22 goals in 10 league's matches rating 9 fixed. I never got him out main 11 and in the second leg of the league he dropped to score 7 with 5 goals in 10 matches despite reaching the 8* mark. Same went on for all my key players i never rested. Give them couple of days without playing (1 day full rest on just training, 1 day starting fdrom bench using them only second half), they'll get back.