Hey, this looks familiar! It's basically the same thing that Chelsea did against Barcelona in the first leg of this tie. In the 57th minute, Roberto Di Matteo has made his second substitution of the afternoon, bringing on Salomon Kalou for Juan Mata.
Mata is an extremely skilled player who is helpful when you need to break down a very defensive team or when you're trying to keep the ball. Chelsea are not attempting to do either of these things. As evidenced by their two goals in this tie, their best route towards goals at the moment is kicking the ball into a space where the Barcelona defenders aren't, then running very fast to that space. Salomon Kalou is much better at running fast than Juan Mata. A logical sub!
We'll have live coverage throughout the game in our Barcelona vs. Chelsea, 2012 UEFA Champions League StoryStream. For more on the two teams, head over to Chelsea FC blog We Ain't Got No History and FC Barcelona blog Barca Blaugranes.