Board Thread:Boruto Discussions/@comment-37077018-20190313103402/@comment-4912646-20190313124154

Sai was called Sai Yamanaka in the novelisation of Boruto the Movie (and probably other places, but that was the first). Characters are never given surnames on the wiki unless they have explicitly been called that in some official media, hence why you don't see things like "Tsunade Senju" or "Karin Uzumaki".