This isn't one of those English Premier League matches where the referee lets a lot of contact go. This might have something to do with the fact that this UEFA Champions League match, despite being contested between two English teams, is not being played in the English Premier League. So, there's no Howard Webb or Mark Clattenburg to let boys be boys. Instead, referee Olegário Benquerença (what a mouthful that name is!) seems to be a little bit more of a 'letter of the law' type of official, not letting any soft fouls go.
These are two great professional football teams and many of these players have played in various leagues around the world, so they should be able to adjust to the different style of officiating. Thus far, a lot of players have been whistled for fouls during challenges that would just be called good, hard, physical play by a lot of English referees, and they'll have to change their game.