My Husband Had His Liver Resection Two Weeks Before. He

my husband had his liver resection two weeks before. he had delayed drainage around a week after surgery. initially the drainage was 3 liters per day but now it is reduce up to 500 ml. He is suffering fever as well mostly 99 degree C in the morning and 100 to 101 in the evening time. kindly advice what could be the reason of fever ?