Guard or guard liners? If it’s the black guard liners, the black plastic section inside the guard, you might be able to do a minor repair using some glue possibly, or you can get new liners at around $60 each. You will need 2 obviously, 1 each for the left and right side, if they both need changing. You may also need to get some more clips to hold them in place depending if you are missing any. These can all be sourced from any Subaru dealership parts department.
I have just done both of mine as I had the same issue. Mine needed changing due to age of them, nothing to do with wheel size in my case.
If it is your guard liners have a look
HERE post number #204, this is how I did mine.