After letting Luis Suarez go in the summer, Koeman is regretting not bringing in another striker to replace him.
"I've said many times that we're lacking one," Koeman said on the need for a striker. "It's the most important thing for the club in January. I don't want to force it, because I arrived knowing the financial state of the club.
"If it's not possible, it's not possible and we'll work with what we've got."
Barcelona's younger players have stepped up this season, with Ansu Fati and Pedri some of the top performers in the squad.
"We know that we have quality young players," Koeman said. "It always depends on the performance of these players. As a coach, I have to protect the young players.
"Pedri is a boy who can't play every game. For them, it's a lot physically. Young players are here to change things at the club. They deserve to play."