Everyone responds to this differently. There are posters here that have had very radical surgeries that I have talked to on the phone, and they are very understandable. Some of them did have to go through speech therapy for awhile, but like many things, for some people it will require new learning.
There are only certain things you can actually control. Your first job and challenge is to get through whatever they decide is the best for you, and come out the other side cancer free. Worrying about what your future will after that is certainly normal, but this is not useful use of your time right now. One challenge at a time. successful treatment, dealing with the immediate side effects and afterward there will be a period of healing.
For everyone that period seems to go on forever, and the associated swelling and more will make your speech poor, but don't think that is your final result. As that swelling begins to subside over a couple of months, you will start to get a sense of where you are in all this as far as how easy it is to speak and be understood. If it needs work.... you have a new challenge, new therapy, and new obstacles to work around and past. Just try for now to focus on the main thing which is getting through the disease process. After all, you have to be around to have speech (and other) problems as my radiation doctor told me..... and he was very good at keeping me focused on one challenge at a time. The mental aspect is more than enough challenge even for the most robust people - let alone the physical aspects of it all.
You eat an elephant one bite at a time, not the whole thing at one sitting.......