My o2 sensor was damaged

Help! I am a first time audi owner and have had my car for 2 years now. The gas mileage has dramatically dropped to 19 now. My custodian (I'm a teacher) had in mind to fix the gas mileage, so has changed the spark plugs and filters using original parts. In the process of doing all this, the o2 sensor was damaged and the dealer said that didn't matter. Is that true that it doesn't metter?
Thanks.