Barça saw their spending severely restricted in the summer due to their ongoing financial struggles, with Ilkay Gundogan and Iñigo Martinez arriving in free transfers and Oriol Romeu brought in for a cut-price €3.4 million.
Portuguese pair Joao Felix and Joao Cancelo also came in on loan, but there was no big outlay despite some significant sales and with the LaLiga-imposed spending budget of €270m for the season already exceeded, Barcelona will need to bring in more money before they can think about signing new players.
Having gone slightly over budget already (€285m), Barça will only be able to spend 50% of any money generated or saved.
"It will be difficult to sign anyone in January," Deco told Mundo Deportivo in an interview.
"We are always ready in case an opportunity presents itself, but I don't think it will be possible."
A deal to sign Brazilian forward Vitor Roque from Atletico Paranaense has already been agreed, but even that may now be pushed back until the summer.
"It doesn't solely depend on the efforts of the club," Deco said. "There is a part that does, but there is also a part that doesn't.
"Vitor is a player we have signed for the present and the future. We really believe in him, but we will have to see what we can do.
"We have to wait and see how we can manage things. If it's not now, it will be in July."
More Barcelona stories
Barcelona reportedly still owe €200 million in transfer fees, which includes money for players who left the club a long time ago, such as Miralem Pjanic, Junior Firpo and Emerson Royal.
Sergi Roberto is out of contract at the end of the season and the right-back could join former Barcelona team-mates Lionel Messi, Jordi Alba and Sergio Busquets in MLS.
Meanwhile, Barça's 16-year-old sensation Lamine Yamal has been likened to Lionel Messi, a comparison which has not gone down well at the Catalan club.
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