The knockout stage of the Champions League features a number of great matchups, and each of the 16 remaining teams will rely heavily on individual performances to advance to the later rounds.
For every remaining club, there are certain individuals -- let's call them X factor players -- who in large part hold the key to their team's success in the tournament. This is not necessarily the team's most talented player; instead, these key players are recognized for either highlighting the team's strengths or hiding any weaknesses via their exceptional play.
Here is a look at X factor players for each of the 16 remaining Champions League clubs. Teams are listed in alphabetical order.
Arsenal - Aaron Ramsey (Midfield)
Injuries to holding midfielders Mikel Arteta and Mathieu Flamini throughout the 2014-15 season have made Ramsey's defensive contributions more important, and so far the midfielder leads Arsenal with 13 total tackles in Champions League competition. Eight of those 13 tackles came in Arsenal's group-stage victories, and by providing a stabilizing presence in midfield, Ramsey opens things up for creative midfielders like Santi Cazorla and Alexis Sanchez to provide more in the attack.
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