Sociedad are preparing for their first Champions League campaign since 2004 after finishing fourth in La Liga, but Prieto has warned them not to let this affect their league form.
The 30-year-old made 33 appearances for the club last season, but has only started one game so far this season.
However, Prieto wants the Basque outfit to make a top-four finish a regular achievement.
"We are clear that Saturday’s game comes first, we have started preparing for it from today," the midfielder said.
"Every year is different and so you never know, we will fight to be as high as possible.
"Now it is not worth putting up a target, but we have a good team and hopefully we can have a good season and again fight for Europe.
"But last season, the Champions League was only a goal in the final 10 weeks of the season.
"We are all excited about the Champions League, we will meet Manchester United and get to play at Old Trafford, but we know that next year, if we want to play again in the Champions League, we have to do well in the league.
"We understand what is important and that where we need to give our highest level is in the league."
Sociedad have won just one of their opening three La Liga games and will be hoping to push themselves back up the table when they face Levante on September 14.
