Borussia Dortmund need to bolster their squad during the January transfer window, coach Thomas Tuchel has said.
Dortmund have allowed two players to leave the club this winter, but, so far, have been unsuccessful in reinforcing the team.
Adnan Januzaj has returned from his loan to Manchester United while Jonas Hofmann has been sold to Saturday's opponents Borussia Monchengladbach.
Under Tuchel, Borussia Dortmund have returned to form after a difficult 2014-15, and go into the second half of the season still battling in three competitions.
They have qualified for the knockout stage of Europa League, the quarterfinals of the DFB Pokal, and also are on course to return to Champions League as they are currently second in the Bundesliga.
The club's resurgence, however, could see Dortmund play 29 matches until mid-May should they reach the final in the cup competitions.
They resume their campaign with 21 outfield players including the two youngsters Felix Passlack and Christian Pulisic as well as Nuri Sahin, Moritz Leitner and Erik Durm, who have not featured in the Bundesliga this term.
"We've lost two players and I think that we need to keep up the quality, also because of the training quality," Tuchel told Der Westen. "If we want to achieve our goals, we need competition."
In recent weeks Dortmund have been linked with several attacking midfielders, but the likes of Dinamo Kiev's Andriy Yarmolenko have been discarded by local media, while the club maintained their position of not commenting on any transfer rumours.
However, Dortmund have not rejected media reports that they hope to bring in Mainz midfielder Yunnus Malli either this winter or in the summer, when a €10 million release clause in the Turkey international's contract can be activated.
According to Der Westen, Mainz have not decided whether to grant Malli, who struck eight goals in the first half of the season, a transfer in the ongoing transfer period.
Kicker on Thursday said that a January switch to the Westfalenstadion "has become less likely" in recent days.