The defensive duo will miss the Blues’ match away on Wednesday as they look to overcome their Championship opponents and set-up a quarter-final against Manchester United
Benitez revealed former Bolton man Cahill is currently suffering from a virus, while Spain international Azpilicueta has a leg problem.
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Chelsea reached the fifth round after overcoming fellow Premier League side Southampton and League One outfit Brentford, but only at the second attempt.
Should Chelsea win at the Riverside Stadium against their Championship opponents, they will take on Premier League leaders Manchester United at Old Trafford for a semi-final berth.