Why would you believe that explanation though? If you had zero knowledge of the human body and muscle growth you might buy it but I think most on here are pretty into sport and open to constructive and fair debate?
My issue with the 'bulking up' story is that it's just not plausible. If we assume Lukaku was on the optimal diet and training program, to allow his story about bulking up for the WC to be true he had a about 35 days to pack on an incredible amount of muscle. If you look across a number of reputed fitness websites (bodybuilding, Livestrong, verywellfit) you'll see the calculation for how much muscle you can gain in that amount of time is realistically about 2-3lb (just over 1kg) unless you are doping. Therefore, and this is giving Lukaku the benefit of the doubt that his diet and training were Olympic levels of perfect, would that amount of muscle really hamper your performances the level he is claiming?
Now, either Lukaku is exaggerating the amount of muscle he packed on and actually just came back a bit out of shape OR he's not human. Judging by his appearance, as many posters have pointed out he seems to be carrying a decent amount of extra timber and, whilst there may well be some slightly stronger muscles under the extra weight, the majority of any extra weight he was carrying post WC must be body fat.
If you're his PR guy and the choice was 'bulking up' or 'letting yourself go' over the summer, I know which one I'd have chosen.