I had exactly this situation.
The plumber knew I was a DIY'er, and suggested that they put some rough concrete in the hole, and let the amateurs finish the job.
I got some "thin patch" from the hardware store (also called "floor leveler") and followed the instructions on the bag.
Before you get started, chip off any bits of concrete that extend higher than the edge of the hole, using a mallet and a screwdriver. You also need to practice "feathering" the edge; basically scraping the surface gradually around the edge, so it doesn't make a hard line.