#4 Kieran Tierney: Celtic FC
A player that has been rumoured to join Manchester United over the past, Tierney may just be the player that United need the most. A weak left back situation has plagued Manchester United ever since Patrice Evra left, as Luke Shaw just isn't up to the grade. Having gone through last season by playing a makeshift ageing Ashley Young as a left back, Tierney may just be the solution for Jose Mourinho.
Mourinho has been showing an affinity towards buying young, but solid players ever since he took over. He has reflected that this season too, and Tierney will be another great addition to this squad. A 21-year-old left back, he has been instrumental to Brendan Rodgers's Celtic side. He played 40 games in their Invincibles season and featured 55 times last season.
He also played every single minute of Celtic's Champions League group stage games, getting an assist for his troubles. He already has a winning mentality and experience playing in Europe, and that makes him an even better fit for Manchester United. If Mourinho chooses to pay upwards of a modest 10 million for Tierney, the young Scot can become a game changer in the United set up.