First of all, I wish all fans that got attacked are well. I am ashamed that has happened and I really thought it wouldn't happen, considering it didn't happen in Serbia in quite a long time (ask Liverpool, Manchester United, Spurs, Glasgow Rangers fans, just to name a few, none had any problems as far as I'm aware). It seems like it was unprovoked and organized attack. Those idiots are probably frustrated that Red Star is having a sh*t season and that's why they wanted to fight. Cowards...
As far as the match goes, both coaches rotated their teams, especially Guardiola. Good luck to you and hopefully it will be a good game, although this match is pretty pointless for both teams.