We talk about Lionel Messi a lot on this blog, for obvious reasons. His ability to change a game - any game - is unfathomable, so it's a given that he's a key player. Here's others you may not have thought about...
It's unclear where Mascherano will play, but he will definitely play. If Sergio Busquets is deemed healthy enough to start, Masche will play in central defense. If not, the Argentine will play in midfield. Either way, his performances go a long way towards determining the result of big games. His height is an issue if he plays in central defense, so in some way it's better that he's more likely to start at pivote. His ability to build from the back will be tested. At one point he was mostly just a destroyer, but his time at Barcelona has brought out the best of him in terms of passing and vision. Even still, he does not measure up to Busquets in those departments. The question is if he can provide enough defensive stability to make up for it.
Barcelona's defense is a hundred times better when Pique is on form, and for the most part this season, he's been in top shape. The Blaugrana conceded from a set piece in the last Clasico, and it will fall largely on Pique to clear the aerial threat. Considering how good he was against Atletico Madrid, one of the top teams in the air, we know he is able to be up to the challenge.
Ivan The Terrible was a Russian Tsar whose name is misleading. The word "terrible" in this case uses an outdated meaning. It did not mean "evil" or "bad", it meant "causing terror." That's exactly what opposing defense have been facing from Rakitic this season. His form has been up and down in his first year at Barcelona, but it's definitely on the upswing right now. Tactically, he's perhaps the most important change from the team that lost the first Clasico. With his ability to do everything - defend, pass and score - he will be a key piece for the Blaugrana.
Another player who has picked up steam recently, this Suarez will present a different challenge than the one Madrid dealt with in the first meeting. Fully fit and well-accustomed to his teammates and the system, Suarez will be an even bigger headache for Los Blancos than he was in the first encounter. That was his very first in Blaugrana colors, and he still managed an assist.