When you are struggling at the wrong end of the table, you need players who will roll up their sleeves and go to war. Would you put your faith in Mirallas or Schniederlin to do that?
In the position that we are currently in, struggling to get results, Niasse is ten times the player that Mirallas will ever be. Despite his limitations as a footballer, you can pretty much guarantee that he would run through walls to score goals and would crawl over broken glass just to play. That is the sort of attitude that pulls a team out of trouble. That is the sort of character we need right now, not half arsed prima donnas.