The main reason why he'll be kept being chosen over Martial is his movement. Saw that yesterday, watching post-match analysis today confirms it pretty much. He's constantly attacking space, something Martial doesn't really do, he wans the ball to his feet, he waits for it. Rashford makes numerous runs even if he's not picked up by the passer. Even if he doesn't receive the ball, he's actively taking part in the offensive play because he's taking a defender with him, making the defender switch his stance, worry, make a decision whether he should go with him or stay with his man. I bet Jose loves that.
Even if he doesn't score that too often he's doing immense off-ball job at times and the goals will come imo, especially when he's going to work with Lukaku now who's a great finisher and hopefully Zlatan when he comes back from injury. He's still 19, he'll work on his finishing but imo he's got the more important part of the game on his side already, one that often is tougher to learn. Combined with his raw physical attributes, it's going to be great. Keep playing him on the wing imo, he'll be brought into the game enough times in a system we played yesterday.