Defeat in the Champions League to PSG was followed by a poor draw against LaLiga minnows Cadiz.
Koeman said: "We know that in two days everything can change, it can change people's feelings, opinions of the press, we have to accept it.
"I think the result of the other day of PSG and Sevilla has hurt us. We are Barça, we have to be proud to play and be here. It is a time where the experienced people, the captains, have to create an environment where we go for everything.
"We have to play our game and put energy that we have put into other games, showing that image is important."
He also said: "I think we can fight for the championship. It is showing that each team can go down, it costs them their matches. It depends on the two games that come, the two that come are important to fight for the championship."