On Saturday the San Sebastian club host fifth-placed Malaga, who are just one point behind them in the race to compete in Europe's elite competition next season.
And full-back De la Bella believes that victory over Manuel Pellegrini's side would be a giant step towards their objective.
He told reporters: "It's a very important game, it's vital, because we are playing against one of our rivals for fourth spot and we could open up a good gap. Also, we have to play our next two games away from home.
"The team are showing that they are going to fight to stay in the position we are in at the moment. It's going to be very difficult, but we have been here for the last two weeks and now it's no longer a dream, it's a reality, and we have to finish the job," he said.
Malaga face Borussia Dortmund on Tuesday in the second leg of their Champions League quarter-final tie, a match De la Bella said was "the biggest game in Malaga's history".
However, the Andalusian club's coach Manuel Pellegrini insisted that his players would be fully focused on their league duties.
The Chilean told a press conference: "I don't think anyone is thinking about Tuesday at the moment, we are thinking about tomorrow [Saturday]. We'll have to think carefully about which team will play tomorrow, but those who are in the team are going to be thinking about playing Real Sociedad."
Pellegrini ruled out using the game against Philippe Montanier's side as a rehearsal for the trip to Dortmund, in which they will be without captain and centre-back Weligton.
"I'm thinking about the game against Real Sociedad and the best team possible will play," he said.
Pellegrini will have a full squad to choose from with the exception of the injured Eliseu, but Montanier has a number of injury concerns.
Xabi Prieto is a serious doubt for the match owing to a thigh injury, while Carlos Vela, Chory Castro and goalkeeper Claudio Bravo have not yet been confirmed fit.