Chelsea won the Premier League last season, but are struggling to finish in the top four this time around.
"I think they realise this season is not easy like last season, because last season they were very focused on the Premier League," Gallas told Goal.
"They didn't have to play in the Champions League or Europa League so it was easier for them.
"This season they have to qualify for the Champions League. It won't be easy because you have got Manchester United, who are second at the moment. You have Tottenham and Liverpool who are very close [to Chelsea].
"You need some leaders when you don't play well, you need some players who can shout on the pitch. That's why John Terry was such a good captain, because he was shouting when we needed him to shout and we were listening to him.
"Even when he was younger than the other players, he was a leader and we respected him. He was the first guy on the pitch, the first tackle. It was a good example for us. It was like when you go to war, you have to know you can go to war with this soldier.
"When you see the soldier on the right side who is not scared to fight, then you want to fight with him. I think in football it is good to have the quality players but when the game is a little bit more physical and difficult, then you need that soldier. That's why it is important to get a soldier like that in your squad to shout and show you where to go."