Funny I had the same thing happen to me. I had a defender named McFaydern and this season he kept performing a 5 or 6. I tried to raise his morale but the same damn BS happened. I was lucky to get a 3 star corner specialist defender who quickly leveled to a level 4. I immediately changed McFaydern's number from 5 to 30 and gave his old number to the new player. xD
Does anyone have an idea why this happens? Even my 27 year old TWO star striker gets a goal or two every game, if not an assist. Again, he's Two Stars and seriously old. >_<