I don't quite know if this fits your criteria but I would join as a box-to-box midfielder and I am saying this as the right player can massively improve our team. One thing we are missing from last seasons is the Vidal attitude. Khedira played well at times but he's far behind what Arturo brought to the team in his best days, not so much in his last season. We need a bulldog in midfield, a player who has so much determination and ambition that inspires teammates to perform above their normal level. A player like this, besides being better than the one he replaces in the first 11, will make his teammates play better, play with confidence. Khedira is injury prone and he is protecting himself in a way Vidal never did, Asamoah also, while Sturaro isn't that good of a player to be in the starting 11. We need a player that we are all sure he will give 101% every game he plays, someone with incredible workrate. We should consider very carefully Khedira's situation because I don;t want another Llorente case...
Sure, there are other obvious positions in which we need better players, but I wanted to expose a big picture perspective, because the mental condition, the attitude, is more important than a player being some upgrade on a player we already have.