Some of the comments on here are bizarre, maybe acceptable if you were at the match and saw that "sitter" only once in real time. He was was clearly nudged off position by a more experienced defender, wasn't a foul just good defending which at this age DCL doesn't have the physicality to cope with. You can see it quite clearly in replays, his body and head are in a diagonal line and he heads it the way he's facing . It's a good chance missed , and you'd expect an experienced PL striker to have got it on target despite the defender, but he's not an experienced PL striker and it wasn't as clear cut a sitter as The opportunity Ross scored from.
DCL is actually quite a good header of a ball ..... for his age. The evidence is there in the goals he's scored particularly his first for England U20 and the header he won and flicked on after coming on. But he's too easily dominated by experienced defenders e.g. Sakho, I think, very much had him in his picket for the RS in the U23s derby.
He's very much a work in progress , and I would say there are better young strikers than him around e.g Tammy Abrahams the Chelsea product at Bristol City, but very very few get game time in the PL at 19 these days.The reason he's getting game time is we just don't have anyone else in his role to support Lukaku, the closest are Niasse and Kone and to answer another question hes without doubt the best young striker we've got in the U23s.