Ditra uncoupling membrane coverage
Construction supply stores in my area just sell ditra by the linear foot (the MSRP is around $6/LF or $2/sqft for the 1m-wide version), so you can just go in and buy a small amount.

If you're relying on ditra for waterproofing it'll obviously be no good at that specific area. If moisture somehow goes through your grout and seeps under the ditra, it'll wick out through your plywood/subfloor anyway so it's not like it's sandwiched or anything. The height difference is easily addressed by using a taller trowel in the uncovered area, and for an area that small, I don't see it as enough to break everything else, even if it's grouted.