I didn't have to replace flooring under a slide out but I had to replace a 6'X4' opposite side.
If your roller is mounted in the floor then you will probably have to replace a larger area so that the edges of the new piece align to the structure underneath. Some of my rollers are mounted in the floor from underneath and some are in the slide. Mine is not a flat floor slide.
In my case I had metal tubing running side to side, can't remember the spacing but it was more then 16" C-C. I had to cut out to where the edge lined up to the tubing. My flooring was a double layer of 5/8" OSB, it crumbled when I started to remove it, I had go until I got to good flooring. I replaced with 3/4" pressure treated plywood, I painted with and exterior primer before putting down. I then used a piece of exterior siding, which was a very dense hard material and had a 50 year guarantee. My flooring did not go under the side wall so it was easy for me to remove.
If you flooring doesn't go under your side wall I would think you can remove/replace without removing the slide. You will have to get creative on cutting the old out, possibly a reciprocating saw with long blade.
2002 Monaco Windsor 38 PKD Cummins ISC 350 8.3L
2005 Jeep Grand Cherokee w/5.7 Hemi