"When you play against teams like Manchester United or Tottenham we have to be ready for the fight, and all the games that we have lost we weren't ready," said Gallas, who has seen Arsenal lose at Sevilla, Middlesbrough, Tottenham and now United this term.
"I'm sorry but we just weren't ready. We have to change our mentality. When we are on the pitch we have to fight before we can play good football.
"When we play away we have to fight, and we didn't fight on Saturday. Winning the Premier League and Champions League are dreams, but to get those dreams you have to fight.
"Tomorrow it will be different as we play at home and we have to play our game."