Lukaku is the kind of striker that scores an hat trick when you are beating a team 5-0 while I prefer the guy that scores that lone goal in a 1-0. In 31 EPL games, Lukaku has failed to score in 16 games, with about 30% of his goals (7/23) coming in 2 matches and almost 60% (13/23) coming in 5 games (which his team scored 3+ goals). Not United starting material in my opinion.
Icardi is the most clinical (he is lethal in the box) while Belotti is the most interesting (he is like a bull in a china warehouse which suits EPL style)
You are looking way too much into this. If you check any PL striker record it will be same. No player will score in all games. Even when RVP was in beast form he didn't score in 10 games on a trot.
Just to compare with other strikers in other leagues.
Lukaku scored in 15/31 games
Kane scored in 11/23 games
Costa scored in 16/29 games
Sanchez scored in 12/30 games
Aguero scored in 12/25 games
Belotti scored in 16/28 games
Dzeko scored in 18/31 games
Higauin scored in 15/31 games
Icardi scored in 12/29 games
Suarez scored in 18/29 games
Ronaldo scored in 13/24 games
Griezmann scored in 12/30 games
Lewandowski scored in 14/28 games
Auba scored in 17/26 games
Few players have better average but in general Lukaku's scoring rate is on par with other big strikers.