Barcelona travel to Alfredo Di Stefano stadium to take on Real Madrid in their upcoming La Liga match, and the Blaugrana will be hoping to extend their 19-game unbeaten streak in the Spanish top-flight and keep pace with league leaders Atletico Madrid.
Koeman also believes Real Madrid have had too much unwarranted criticism this season as he pointed out Barcelona's El Clasico rivals are in the Champions League and La Liga title race.
"A Clasico is a different game to the others. Barcelona are doing well, [we] are on a good run, we are fighting to win," Koeman said ahead of El Clasico.
"Madrid have had too much criticism; they are in the Champions League and fighting for LaLiga. I'm not a supporter of the people who have criticised [them]. They have great players and it's a great match. Both teams know the importance of winning."
Koeman also claimed the outcome of El Clasico won't be decisive in La Liga title race, but admitted the winner will 'have an important morale boost to build on'.
"The result is not decisive, because after tomorrow's game we still have many games to go and it's difficult for any team to win all their matches," Koeman said. "The team that wins [the Clasico] will have an important morale boost to build on."