For the first year or so of all grain brewing, I only had one burner and kettle. I would mash and batch-sparge as follows:
1/ heat mash water in kettle; add to grains in mash tun.
2/ heat sparge water in kettle.
3/ transfer sparge water to bucket.
4/ drain mash runnings into kettle.
5/ sparge water -> mash tun
6/ combine sparge with mash runnings in kettle
7/ boil, &c.
So, yeah, if you don't have a separate vessel, you'll split the effort between the kettle and a bucket or some other container, which is – effectively – your HLT.
EDIT: or as Bobby_M notes, swap the kettle/bucket roles. 6 of one, a half-dozen the other way.