Rashford is pretty easy to play against, I don't think you're going to get opposition fans fearing what he might do whenever he gets the ball. Rashford has gone through the same progression that Rooney did; started off as a raw, exciting prospect that was physically brilliant and would scare defenders, but then after a few years, mature into a more 'sensible' player. The big difference is that when Rooney lost that raw edge, he was still quality because he was intelligent and technically gifted whereas now that Rashford has gone through the same thing, he's not got enough substance to pull it off.
Giggs was the same, once a very raw, exciting prospect who terrified defenders but when he matured, he was able to still be as much of a threat. At the very end of his career, he was able to adapt again and because of his technical quality and knowledge of the game, still became an asset, dspite all his pace going.
I just don't think Rashford is technically good enough to adapt and become a quality player as he matures. There are definitely issues with his footballing intelligence and how he reads the game too, so now that he's lost that raw, exciting potential we saw under van Gaal and Mourinho, it's hard to see what he actually offers now. Fans of a player of his age at a club like United shouldn't be wishing they had the 19 year old prospect back.