The Basque outfit occupies the last Champions League qualifying spot heading into Monday's game at Levante.
Athletic have won one of their last five league games and their 2-1 home defeat to Primera Liga leaders Atletico Madrid last time out allowed nearest rivals Real Sociedad and Sevilla to close to within six points.
De Marcos believes his side cannot take anything for granted with seven games remaining.
"We are going to have to win many games to finish fourth," he said. "We are in the race to finish fourth and we want to continue to defend our position.
"The game against Levante is very important."
Athletic have not lost on the road since January 5 and will be looking to inflict Levante's third-straight defeat at the Ciudad de Valencia stadium.
Levante halted a four-match winless run with a 2-0 triumph at Granada in the last round.
The outcome allowed the Valencian outfit to climb to 10th and reach the 40-point mark, a figure deemed sufficient to avoid the drop.
Levante coach Joaquin Caparros is nevertheless cautious and believes his side needs another victory to feel completely safe.
"We are almost there," Caparros said. "But in football that "almost" is the hardest step to make.
"We have to continue to work hard and try to hand a good performance against Athletic."
With Athletic captain Carlos Gurpegi out injured and defender Aymeric Laporte suspended, Unai Albizua is set to earn a start against Levante.
Albizua has played just 32 minutes this season.