It's the best option this window. If there was no available LW that conviced the club and Flick 100%, it's better to seek a short-term solution via one season loan and try next window again.
Rashford will do his job against LaLiga's Mallorcas and Levantes, especially in Flick's system. Very good decision instead of spending much money on a player you possibly regret after a season.
There is also a chance of La-Masia providing a winger going forward. Virgili and Pedro Fernandez so far made a big impression on Flick according to reports. This would be the best case anyways.