At the club's AGM on Wednesday night, Del Nido revealed a budget deficit of 15million euros which is forecast to grow over the coming seasons unless action is taken.
Describing the results as "the worst of all my years", Del Nido said: "We took some risks to try and take Sevilla to the top despite having a lower income.
"The deficit is the result of our elimination in the Champions League qualifier with Braga (in 2010) and losing to Hannover in the Europa League qualifiers (in 2011), and then not qualifying for Europe last season."
He added in AS: "We have done badly, we have not done things the right way and have spent two years making mistakes.
"We need to sell players or return to the mediocrity of the past. What I want is to continue to aspire to something and not just wander through the league unnoticed."