The Denmark defender could be cashed in by Chelsea this month as he struggles to win over manager Frank Lampard.
Christensen enjoyed two successful seasons on-loan with Gladbach and Eberl says they've kept in touch.
He told Express: "We don't need to talk about the quality of Andreas Christensen.
"I do not know whether it (a transfer) could be implemented. I have not spoken to Chelsea. The fact is that Andreas still has a two-year contract there.
"If we then get to know what transfer fees are also required in England for players who may not seem to play a major role... it's crazy."