The Belgium international has been linked with a loan move away from Stamford Bridge in January following his failure to secure a regular place in the first team.
"We will make an evaluation with the club in the next few days and decide what is the best solution," De Koster told Goal.com.
"He has told me that every day he is training like a demon because it means a lot to him, but I will not say that he has to move in January. We will wait for the evaluation from the club."
Atletico Madrid and Borussia Dortmund have been touted as possible loan destinations for the 22-year-old, who signed for Chelsea from Genk in January 2012.