Patio Sliding Door

Previous homeowner poured slab for rec room he built and outside patio, so it's same height inside to outside. Slider, which is contractor grade leaks at bottom corner. I want to replace door. Apparently, when slab was poured, slider was an after thought so recess for track wasn't factored in. Was told I'd need a pan for new door, but all info I've seen on pans, shows it being used on raised threshold. Any ideas on options?