You do not know how hard it is to perform well when you only get one chance in three months. Bench Costa for 10 matches and I will guarantee you that he will become shit. KDB showed in his pre season and in his first match that he is capable of playing on a high level.Somehow Mourinho did not see that.He never played him again,or he never gave him a fair chance again. Costa played a good game against Burnley,but the difference was that Mou continued to play Costa in almost every game,which gave him more confidence and he could work on his weaknesses,while KDB played a very good match against Hull and after that,KDB did not get any chances,which was a big mistake of Mourinho. In general,youth team players at Chelsea have no chance because Mou has weird ways to integrate them.Even if they play good,he lets players play,who are more experienced,even though some of the experienced players are not good enough. It is what it is.