I drained my pool for a couple of weeks to change the water, clean the
pool surface and replace the pump. Today as I started filling it I
noticed that a 4" chunk of plaster under the skimmer inlet had fallen
away to reveal the gunnite beneath.

Do I need to stop filling the pool below the plaster hole to prevent
water from getting inside, and behind, the plaster? Or is this a
repair I can deal with another time and I can fill the pool up to the
top for now and then just drain the pool below the hole when I'm ready
to fix it?

How hard is it to patch the plaster, is this something I can do myself
or do I need to have a pro do it?