Due to your knee problem it will be OK. An employer would prefer you are at work than the only reason you are off sick is due to a shoe issue.
Get some leather flats that are dressy as these will look nice with skirts or pants. Have a look at ones with a slight/tiny heel as I found these really comfortable after I did my ankle and could not wear heels. 6 months down the track I find I still need to wear the low heels if I have twinged the ankle.
Also have a look at your work dress code. Unless they state you have to wear heels they cannot say anything.
Remember if anything is said just tell them you will get a certificate to say you cannot wear heel at the moment.