Arsenal sporting director Edu has warned the Gunners that they will have problems if they don’t prepare well before facing Porto in the UEFA Champions League round of 16 clash. Edu issued the warning after the Champions League round of 16 draw at UEFA’s headquarters in Nyon, Switzerland, on Monday. Speaking after the draw, Edu told Arsenal’s official website: “We are going to play against a club that is very used to playing in the competition because they play in it every single year and they have a lot of players as well with big experience. “We have to understand how we are going to play and…