In 16/17 Lukaku was joint 3rd highest goalscorer with headed goals. He got 6 goals, joined by Antonio and Giroud and in front of them were Benteke and Llorente with 7 each.
I can also see that he scored those 6 goals from 18 headed attempts at goal. Giroud had slightly less attempts but Benteke had 49, Llorente 35 and Antonio 21. Ibrahimovic that season also had 20 headed attempts converting 4 into goals.
So 1 off the highest in the league but with a lot less attempts. Same can be said of Giroud.
Last season Morata scored the most headed goals in the league with 7, Kane got 6, Aguero got 5 and thats your top 3.
Lukaku was joint 4th highest goalscorer with his head. There were 9 players who all scored 3 goals.
Morata got his 7 goals from 23 headers at goal, Kane got his 6 goals from 29 headers at goal. Lukaku got his 3 headed goals from 19 headers at goal.
So since he's come to United he scores half as many headers as he did the season before and less than the likes of Morata, Kane and Aguero. I dont think Lukaku has been quite as bad as has been made out but even last season his heading conversion was worse than in the past.
So far this season Lukaku has scored 2 from 6 headed attempts and Mitrovic has the most in the league with 3 goals from 14 attempts. Kane also has 2 from 8 headed attempts.