The two clubs has nearly reached an agreement, although there are some details which still need to be finalised.
"I am waiting for information from either Leverkusen or Chelsea, but there could be some positive news by the end of this weekend," agent Ingo Haspel told Sportal.de.
"It is an indication of how much Chelsea want the player that they are continuing to negotiate."
The main stalling point appear to concern Chelsea's Kevin De Bruyne, who is on loan at Werder Bremen.
Leverkusen want to sign the Belgian on loan as part of the deal but several clubs are interested in signing the young midfielder.