Chelsea did brilliantly out of Lukaku - £28million wasn't it? He basically paid for Fabregas.
He turned 21 in May, in 19 starts and 3 sub appearances he has 7 goals and 3 assists. Everton have had more injuries and a tougher season with more appearances in a shorter space of time. Any smaller squaded team will 99% of the time do worse in general comparing a season without european/long cup runs vs a season with europe and maybe more cup games as well.
He's young and hasn't played at that top level 60 games a season rate and he had a longer summer to go with it after a great season. he's doing absolutely fine, danny just hates him. There is no young striker in any league who has scored as well as him in the past 3 years, he won't score 15-20 league goals every year, but he has done that for two seasons in a row before turning 21. There is a reason why guys like Ronaldo, Bale, Suarez(to a lesser degree) Messi all get better and more consistent at 23-24, and Ronaldo/Messi have gotten more consistent since then and towards being 30 as well. 24-30 year old players at top teams tend to cope far better with european football and cup runs and playing closer to 70 games a year than 45, teams that are consistently in europe are also more consistent with that number of games. If Everton get far in the competition and he played 50+ games this year, and again next year he'll do better, if they do it again he'll improve more until like other older players he becomes more accustomed to playing 50+ games a year.
Only people who are expecting WAY to much because the price is factoring in how good he could be IN THE FUTURE, are disappointed with him in a tough year for a young striker.
There is no striker with the level of potential he's showed in europe currently. He could still easily finish this season with 15+ league goals. 28million for a 21 year old who scored 15+ in the past two seasons is a snip and a tough first year for this squad in Europa league doesn't change that.