I like Marcus but he's not in the same league as Rooney.
If Rooney was going through a bad patch form wise, even within an individual game, he'd get angry and redouble his efforts. The street fighter would come out in him. Sometimes too much, he'd fly into tackles and probably could have got himself sent off a few times. But at least he was doing something!
When Rashford is not playing well, everything breaks down with him. His confidence goes, his energy levels drop, he can't handle any physicality, he won't challenge for the ball.... and because he played so well earlier in the season and scored x amount of goals, he's almost immune from being substituted, no matter how ineffective he's been, on the off chance that he might, maybe, possibly, produce a moment of magic.
Most fans can forgive him going through a bad spell form wise, but you don't have to be playing well to put in a tackle, chase down defenders like your like depended on it, challenge for headers. I just wish he'd do that when he's not playing well, that's all.