Heart Patient - Father aged 70 years.

Dear doctor, My father aged 70 years, recently for aorta burst dissection on 19th Jan.,2022..It was a traumatic situation for us .The cardio team of doctors did Bentall Surgery, to revive him back for life .On 4th Feb, he was detected with mild COVID 19 Infection, with mild fluid in lungs on left lungs, which the doctors are treating with antibiotics, there is improvement in his Chest X ray report.. Our main concern is that our father is getting fever , post operative care since 19th Jan, he is under constant medication by doctor's team..It's been over the month since low grade fever in evening time is persistent..It is post operative side efects or COVID 19 side effects .Is it normal to have frequent fevers at evening time daily 99-100 degrees..Pls advise..It's important to understand the gravity of cause behind it..Thank you..