If you can't afford a new plywood substrate for your roof consider this DIY option.
1. Dry out roof completely as possible after removing all trim.
2. Drill multiple small holes thru damaged plywood. Try not to drill thru the entire plywood sheet just enough to get to the final layer but not thru it.
3. Inject low viscosity epoxy into holes. Use a marine grade epoxy that is compatible (as much as it can be) with wet wood.
4. Put heavy object on damaged area to compress playwood back together as much as possible.
5. Apply appropriate glue to plywood and glue roof to it.
6. Apply widest available eternabond tape over seam between roof and cap.
EDIT: LOOKING AT YOUR ROOF WOOD, IT LOOKS LIKE OSB AND NOT PLYWOOD.
It doesn't look like the OSB is bulging but I'm sure it is a little soft.
In lieu of epoxy, I'd use thin Cyanoacrylate glue which is activated by moisture in the wood. Be careful, the heat of activating when curing especially in face of mositure can cause skin burns.
Cut the rotted wood out install new wood sexction: Not that difficult: See video: