Gary Cahill did not travel with the Chelsea squad for the first leg of their Europa League semi-final against Eintracht Frankfurt.
Speaking at a news conference in Germany of the eve of the game, head coach Maurizio Sarri said the former England defender was unavailable because of injury.
Chelsea are also without the injured Antonio Rudiger, who will miss the rest of the season with a knee problem.
Andreas Christensen is therefore set to come in to partner David Luiz at centre-back, where Chelsea’s options are currently limited – Ethan Ampadu is also out.
Cahill has been out of favour under Sarri and is set to leave Chelsea at the end of the season.
Sarri suggested that the Blues stalwart might not have played his final match for the club, but also revealed that a back-up plan to deploy Cesar Azpilicueta as a makeshift central defender has been worked on.
Sarri explained: “Cahill is out with a problem with his Achilles tendon. I hope to have him back in one week, but I don’t know at the moment. I have to speak to the doctor.
“So we are in trouble with the centre-backs. In the last two training sessions we played with Azpilicueta at centre-back in case he is the only one that can play there.”
Meanwhile, Willian is in the squad and available to play despite going off against Manchester United after picking up a knock.
“Willian is OK. It was only a knock. He has trained and I think he will be able play,” Sarri confirmed.