Liverpool's top-four hopes were dealt a blow on Sunday when they were beaten 2-1 by Crystal Palace at Anfield.
Wijnaldum refuses to throw in the towel, though, and is determined to bounce back against Watford next week.
"As long as there is a chance to qualify for the Champions League, we have to believe in it," Wijnaldum told the official Liverpool website.
"If you do not believe you can reach it, it is already done. We must have the confidence and stick together, helping each other out in moments like this.
"We will fight for the Champions League spot. That is the only thing we can do now. We cannot change what happened against Palace, so we have to look forward.
"Against Watford we can change everything and get a good result. We have to look game by game and at the end of the season we will see what happens."