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.
Atletico Madrid - Koke (Midfield)
Atletico overwhelmed their group-stage opponents with a steady stream of dangerous crosses into the opponent's penalty area, leading to the team creating a tournament-high 35 chances from those crossing attempts. Koke is the main facilitator with 13 total chances created from crosses, and in the three group-stage instances that Atletico averaged at least 30 cross attempts, Koke produced a team-high three assists and created 10 chances with his accurate service from wide areas.
Barcelona - Luis Suarez (Forward)
Suarez joined Barcelona for the last three Champions League group-stage games, and in that time, Barcelona scored nine of their 15 goals in the competition.
He immediately adds another dynamic to Barcelona's forward line with his ability to interchange with Lionel Messi and Neymar in the team's 4-3-3 formation. More importantly, Suarez added to the attack by recording 22 total touches inside the penalty area in his three Champions League appearances, and domestically the striker leads La Liga with 9.3 touches per 90 minutes inside the penalty area.
Basel - Fabian Schar (Defender)
Basel defend their Champions League opponents by keeping a high defensive line and looking to draw them offside, and the team ranks second in the tournament in provoking offside calls. Center-back Schar orchestrates this cohesive defensive movement, and in two signature group-stage wins against Liverpool and Ludogorets, Schar drew opponents offside five times while leading the team in tackles and clearances.
Bayer Leverkusen - Lars Bender (Midfield)
Leverkusen ranked fifth in the Champions League with just four goals conceded in the group stage, and Bender is proficient at covering ground and keeping the team organized in midfield areas. He leads the team with 19 tackles in Champions League competition, and the fact that 16 of these tackles came when Leverkusen was either tied or ahead by one speaks to his ability to step up when needed.
Bayern Munich - Arjen Robben (Midfield)
Robben leads Bayern in take-ons (a play in which the player controlling the ball tries to beat a defender) and chances created in Champions League play, and the winger's presence on the right side provides balance to Bayern's overall attack. Bayern prefer to build their attack on the central and left thirds of the field and then record the majority of their attacking-third touches on the right side of the field. This structure allows Robben to isolate himself against a minimal number of opposing defenders, and his ability to create and finish chances in those isolated opportunities is critical to Bayern's success.
Borussia Dortmund - Pierre-Emerick Aubameyang (Forward)
Aubameyang has scored two of Dortmund's tournament-high four goals from counterattacks, and he leads the tournament with five total shot attempts from those situations. The striker has also scored twice from counterattacks in domestic competition. Further, Dortmund have won all four games where Aubameyang has contributed a counterattack goal, outscoring opponents 12-2.
Chelsea - Cesc Fabregas (Midfield)
Fabregas ranks second in the Champions League in overall touches and in total chances created. He is also critical to Chelsea because he contributes defensively. The midfielder's rate of three tackles per 90 minutes played a direct role in Chelsea ranking third in the group stage with just three goals conceded.
Juventus - Alvaro Morata (Forward)
Morata has scored and assisted in each of his last two Serie A appearances, and he is poised to play a bigger attacking role for a Juventus team that scored just seven goals in the UCL group stage. In domestic and European competition, Morata combines with lead striker Carlos Tevez more than any other Juventus player, and the 22-year-old ranks second on the team with 10 shots from inside the penalty area in Champions League play.
Manchester City - Sergio Aguero (Forward)
Aguero's five group-stage goals represented more than half of City's UCL scoring output. Plus, the striker's three goals in a critical 3-2 win over Bayern Munich show how important Aguero is to the overall attack. In Premier League play, Aguero's nine goals scored when his team is either tied or losing by a goal ranks fifth among players in Europe's top five leagues, and this timely scoring ability will become increasingly important in the later rounds.
Monaco - Jeremy Toulalan (Midfield)
Monaco led the Champions League with just one goal conceded during the group stage, and Toulalan leads the team in both tackles and interceptions heading into the knockout rounds. The 31-year-old holding midfielder has also appeared in 10 of Monaco's 12 Ligue 1 clean sheets this season, and his ability to anticipate opponent movement and angle them into more backward and square passes allows for more organization along Monaco's back line.
Paris Saint-Germain - Edinson Cavani (Forward)
Cavani's five goals account for 50 percent of PSG's total Champions League output this season. The striker is more aggressive in Champions League competition; not only does he average more shots and take-ons than he does when playing in Ligue 1, but he also averages one more touch per 90 minutes inside the opponent's penalty area.
FC Porto - Jackson Martinez (Forward)
Martinez leads Porto with five goals in UCL play this season, and the team went 4-0-1 in Jackson's five group-stage appearances. Porto's draw came in a late September game when Jackson attempted just three attacking-third passes -- his lowest number of the tournament -- and Porto have become a stronger team as the 28-year-old striker has evolved into a player who can better combine with his teammates in the attacking third.
Real Madrid - Karim Benzema (Forward)
Benzema is Real Madrid's most consistent Champions League scorer, producing goals in four of his five group-stage appearances. More importantly, Benzema needed just 11 shot attempts, and while his five goals are tied with Cristiano Ronaldo for the team lead, Benzema's efficiency around the goal makes him a vital part of the team without cutting into Ronaldo's and Gareth Bale's need for touches. Additionally, Benzema leads Madrid in UCL play with 17 total chances created, and his ability to both score and distribute are critical to Madrid's overall title hopes.
Schalke 04 - Marco Hoger (Midfield)
Defensively, Schalke struggles to close down attackers. As a result, Schalke conceded 13 goals in the Champions League group stage -- the most of the 16 remaining teams. And while the team ranks second in interceptions, it struggles to make timely tackles in critical areas of the field. Hoger currently leads the team in tackles; however, Schalke conceded eight total goals in the two group-stage games where Hoger failed to record at least four tackles, suggesting that his ability to contain opposing attacks will be critical to Schalke's overall chances.
Shaktar Donetsk - Luiz Adriano (Forward)
Adriano leads the UCL with nine goals, and he has demonstrated extreme efficiency in needing just 14 shot attempts to score so many goals. The striker scored seven of his nine goals from within 12 yards, suggesting a strong nose for the goal, but he will need to prove that he can score against world-class competition after tallying eight of his nine group-stage goals against BATE Borisov.