The UEFA Champions League is seriously starting to heat up now, with quarterfinal matchups set and ready to grace world football's stage this week.
We've already seen how two matchups fared, and with two more set to take place on Wednesday, it's shaping as another big week of European action.
For the four sides in question here, and the four who will play their second leg next week, they'll be looking to their key players to get them over the hump here with inspired performances. However, it isn't always those players who will end up having the biggest impact, with certain X-factor players capable of completely changing the flow of the match.
For some teams they're the big names; for others they're not as well-known.
Read on to see the biggest X-factor to watch in every UCL quarterfinal.
Malaga vs. Borussia Dortmund: Ilkay Gundogan, Borussia Dortmund
Ilkay Gundogan isn't an attack-minded midfielder who's going to drive Borussia Dortmund forward and score a plethora of goals against Malaga.
But what he is going to do is hold their midfield together—setting the German side up for a strong defensive performance by trying to hold out the Spanish club when they press forward.
Malaga have a number of good attacking options, and with Gundogan likely to pick up star midfielder Isco, the defensive midfielder could well have a profound impact on this match. If he can shut out Isco, then Malaga's attack will not be as effective and as dangerous as it threatens to be, and that should allow Dortmund an easier night at the back than expected.
Dortmund will be desperate not to concede here and carry an advantage with them back to Germany, even if that advantage is just a scoreless draw.
Gundogan is the player who could make that happen.
Real Madrid vs. Galatasaray: Felipe Melo, Galatasaray
Much like Dortmund, Galatasaray will head on the road in the Champions League this week with their paramount concern that of preserving their defense. However, unlike the Germans, they're playing one of the most potent attacks in world football in Real Madrid.
The Turkish giants will seemingly have a long 90 minutes ahead of them at the Bernabeu, with high levels of pressure and possession working against them.
Enter Felipe Melo as a defensive weapon for the visitors.
Melo has averaged an incredible 3.7 tackles per game and 1.6 interceptions in the Champions League so far, and will look to replicate those numbers in this one.
Against Madrid's attacking strengths, the 29-year-old will seemingly have a number of chances to keep his team in this one. Expect him to chalk up big defensive numbers in the middle of the field, and if he can disrupt the play to Mesut Ozil in the middle, the Turkish champions could surprise a few people in this one.
Barcelona vs. Paris Saint-Germain: Zlatan Ibrahimovic, PSG
With Lionel Messi potentially on the sidelines for their return leg, Zlatan Ibrahimovic will be the striker to watch as PSG try to stun the Catalan club.
Ibra was able to find the back of the net in Paris and was the French club's best attacking weapon all night—both in the air and on the ground. He will surely be the same threat again in the return leg next week and will continue to cause havoc for the home side here.
The Swedish international will be an extra big factor to watch in this one, given the fact that he has played in Camp Nou before with Barcelona. And with Messi the reason behind his eventual departure from the club (who could be absent here), expect the enigmatic striker to come up big in this one.
We've seen it already in David de Gea returning to Spain and Arturo Vidal returning to Germany. Will we see it once more when Ibra returns to Camp Nou?
Juventus vs. Bayern Munich: Paul Pogba, Juventus
Down two goals and missing some key players for the return leg, Juventus will need to make some big changes at home if they're going to beat Bayern Munich.
One of those must be starting the young Paul Pogba.
Pogba showed in the limited time that he was on the field against Bayern, and in the international against Spain last week, that he can be a dynamic presence in the midfield, whose dogged determination can bring about errors and chances at any moment.
Juventus will need a spark in the middle and someone to help Andrea Pirlo move the ball up the field, and that spark has to be the 20-year-old Pogba. After all, they've got nothing to lose and could very well be overrun by Bayern in the middle if they don't look for alternatives.
It would be a risk, but Pogba could just be the key for Juventus here.
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