#4 Angel Di Maria

Angel Di Maria joined Real Madrid in the summer of 2010 from Benfica. Five seasons later, it came as a surprise when Los Blancos sold the Argentinian to Manchester United, albeit for a hefty price tag. He failed in his only season at Old Trafford but was back to his best at Paris Saint-Germain.
Gareth Bale's arrival was one of the reasons why Di Maria shifted to a central role in his final season at the club. Nonetheless, the Argentinian was effective as usual and yet Real turned a blind towards his contributions to the La Decima campaign.
Today, Gareth Bale is having a rather torrid time battling a series of injuries whereas Di Maria is adding accolades to his trophy cabinet successfully at Paris Saint-Germain. Looking at the Welshman might make Real rue their decision about parting ways with Di Maria.
Considering Real's unexpected downfall and Bale's gradually decreasing contribution due to injuries, there's no doubt Di Maria's presence would have made a difference in uplifting them to their former glory.