I'm not sure how they measure that, it seems odd since Lukaku shoots ~2.7 times per game in 35 games and has 23 goals in the league. [From whoscored]. That's 95 shots. 6 of which are penalties.
Gives us 89 shot attempts for 17 goals.
17/89 is a
19% conversion rate. You can check for yourself how many of his goals were tap ins or just simple touches, which makes it an easier goal to score for anyone, and that 19% rate doesn't really look very solid.
But still, the problem is not with the finishing, that's actually something he's quite good at obviously. He can finish, even if at times it is frustrating as you have to expect the GK to mess it up more than Lukaku to succeed given how often he shoots right at the GK.
The problem is with fumbling the ball during attacks, losing possession easily, misplacing passes or just refusing to take the shot as if he cares so much about such statistics when he's not confident he can get the goal! It feels sometimes that we have to try the same thing 3-4 times in order to get it right whenever Lukaku is involved. It will work one of those 3 or 4 times, but it becomes a problem when you can only have a go once or even twice. Against smaller teams it can happen more frequently, so effectiveness trumps efficiency, so people just don't care if it takes 5 times to do something right if you're gonna attempt it 15 times.
edit:
Just for comparison.
Luis Muriel: 20 non-PK goals. 35 appearances, 2.4 shots per game, which is 84 shot attempts, minus 3 PK attempts, that's 81.
20/81 = which is 24.7% conversion rate. Around half of his goals are rather difficult. That's without even going to the per minute stuff....
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