Chelsea assistant manager Gianfranco Zola had to be readmitted to hospital after suffering complications from gallstone surgery.
Zola initially had surgery last week and was not on the bench for Monday's FA Cup fifth-round defeat at the hands of Manchester United.
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Maurizio Sarri's assistant had been expected to return for Thursday's Europa League last-32 second-leg clash with Malmo but he was not alongside his fellow Italian on the bench at Stamford Bridge.
The club is not putting a timescale on his return to training and as part of Chelsea's matchday coaching staff.
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That means Zola, who returned to the club after Sarri's arrival last summer, could also be struggling to return in time for Sunday's Carabao Cup final.
Chelsea head to Wembley to take on Manchester City and try to claim their first trophy under Sarri, as well as a modicum of revenge for their 6-0 defeat in the Premier League earlier this month.