He's not a striker. This has been clear from the very start with Rashford, so it's not as if there's an air of mystery to it. He doesn't have the attributes. Or at least enough of them. If pace was the main criteria for a striker Theo Walcott would have had a mesmerising career. He can do a job occasionally, especially if we're purely counterattacking but it shouldn't be used very often at all but we have to because we have messed up the transfers over the years.
He doesn't hold onto the ball, it's like a hot potato when it goes into him back to goal. He doesn't like physical contact, which is impossible when you're leading the line. He still doesn't really have that instinct to sniff out goals and when he does get a chance a proper 9 would be convincing with he fluffs it (see that header which was offside, but ultimately a demonstration of poor heading technique).
Rashford needs the game in front of him at all times, as soon as that isn't the case he is redundant. If you get him facing people up, making those inside runs for teammates to find, that's when he is a threat. He's better when there aren't many options. When he's on the left the option is isolate a fullback, and cut inside. That's his forte. But as a striker you need way more variety.