The 1st player that Sanchez would have threatened is Ibra, because we have another backup striker. The 2nd player that could be threatened is Mata, considering the form of Lingard and Martial, and Sanchez can easily replace Mata on experience, assists, and goals, and with more pace i.e. play him on the right. The one that would threaten Rashford is himself, if he doesn't learn and progress.
Besides, why do players move on loan - in order to have more game time. If Rashford is already starting 20 games a season plus loads of sub appearance, why does he need to move? If you can't learn at 20 games then you are not learning at 40 games neither.