Hit hard by previous mismanagement and the impact of the COVID-19 pandemic, Barca are in financial turmoil with debts reaching €1.4billion earlier this month.
It resulted in the loss of talisman Lionel Messi, whose 21-year association with the Blaugrana ended in early August when Barca confirmed LaLiga restrictions meant re-signing him was an impossibility following the expiration of his contract in June. He subsequently joined PSG on a free transfer.
Barca are still looking to trim their squad to cut their wage bill. Gerard Pique was among those to take a pay reduction so the club to fall in line with salary cap rules, thus allowing them to register Memphis Depay and Eric Garcia, two of the four new arrivals this transfer window alongside full-back Emerson Royal and striker Sergio Aguero, who is out injured.
With rivals Real Madrid bidding for PSG star Kylian Mbappe, Koeman is still reportedly wanting to add to his squad before Tuesday's deadline, though he accepts Barca have to know their limits.
"Every person who wants the best for this club is frustrated but you also have to be realistic, and economically the club cannot compete with other teams like PSG, City or United," Koeman told a news conference ahead of Sunday's clash with Getafe.
"It is like that and you have to accept it. It will last for a long time, it is not a matter of ending today or tomorrow. I have always said that you have to be realistic and know the situation of the club.
"You have to know the moment of the club, that we are changing things, with young players. We are missing six or seven players who can start but we have a good team.
"I am not in favour of thinking about not winning things. We are a club that must always show a winning mentality, but at this moment we cannot fight with the best in the world.
"It does not depend on the best player in the world, it depends on the team and the mentality. We know that we lack the best in the world and we have to have other things."
Asked for his opinion on Madrid's chase of Mbappe, Koeman replied: "If Madrid have the money... I would also want to have him in my team. If you can improve your squad with these players, perfect.
"It seems crazy to me and absurd figures to pay so much money for a player seeing how the world is."
Samuel Umtiti and Miralem Pjanic are players who Barca wish to move on, but Koeman hailed the latter's attitude and application despite his situation, with both making the squad for the Getafe match.
"There are just a few days to go to release players who want to play for other teams. Pjanic is one of the players. His attitude is very good and that's why he's in the squad list," said Koeman.
One player who could bring the star quality Barca crave is Ansu Fati, who has returned to training after missing the majority of last season with a serious knee injury.
Koeman, though, insisted Barca will take it slowly when it comes to easing the teenager back into action.
"We don't have to go fast. I think Ansu still needs more time to become fit at the best level," said Koeman, who is able to welcome back Marc-Andre ter Stegen and Oscar Mingueza for Sunday's game.
"We are talking about a player who has been out injured for seven or eight months and you cannot recover your good form in three weeks, so I don't want to say one date.
"I read in the press that he would be available against Sevilla [on September 11]. I can confirm he won't be ready for that game, he needs more training sessions because we want to have Ansu for a long time and we are not going to take a single risk."