I also think that it's a good decision for all of the parties involved, Martial, UTD, and Sevilla. Martial has got another chance to revive his career, United has got rid of a player they were not playing as they have a huge lot of options in their attack, and Sevilla has got a young exciting prospect to play him for half a season and decide whether to finally buy him or not as he came in loan.
And it couldn't come at a better time neither. Martial needed to play and the time, United needed to save some cash but more importantly, Sevilla never really tries to win the league. I mean when was the last time Sevilla won the league? I do not even remember it, and not that they are at the top of the league now but they are at least very close to Real Madrid.
With an addition like Martial, if everything they are doing ends up being great and no problems with chemistry and all of that, then we are going to end up with Sevilla fighting for a championship thanks to him. If he scores bunch of goals the moment he starts playing and they also end up being champions, this could go as one of the all time best winter break transfers ever.
Man United speculated when releasing Martial to Sevilla on loan. If Man United sells Martial at this time then Man United will only get a little income because Martial rarely plays and his performance is quite bad, so the selling price is not high. At Sevilla, Martial has the opportunity to play more, and if it turns out that his performances are good there, then Martial's market value will also go up.