For me, the best way is to let him play FREQUENTLY and train him also. I got my GK: Olivier Baumann, from letting in all the goals, to keeping the most clean sheets. I got my FC: Robert Lewandowski to missing an open goal and always missing penalties to scoring 2 per match. So for me , you just have to be patient and the player will be better. I waited 3 seasons for my 'Dream Team'. I hope this helps!!