Baba joined the Blues in 2015 but has endured a miserable spell at Stamford Bridge, making just 15 Premier League appearances for the club.
The Ghana international suffered anterior cruciate ligament damage at the start of 2017, ruling him out for the remainder of the year.
However, he is now on the comeback trail and Schalke, who took Baba on loan in the 2016-17 season, are in talks to secure him for the rest of the season.
"I expect a decision to be taken in a relatively short term", Heidel said to german outlet Reviersport.
"Many things are already exchanged between Chelsea and us. Now I'm waiting for an answer from Chelsea to my most recent email.
"It's not certain yet if the loan can be for 18 months."