I do not have the Sphinx search engine installed. For testing purposes, I intentionally selected Sphinx Fulltext as the active search backend (General --> Search Settings).
I then selected Create Index. Sphinx completed index creation in less than 1 second with the following result:
Number of posts in main index: 0
Number of posts in frequently updated delta index: 2076719
Total number of indexed posts: 2076719
The total post count is correct.
I performed a search using the /search.php search engine, which then displayed with the following error:
Search failed: connection to localhost:9312 failed (errno=101, msg=Network is unreachable)
The issue is that a site administrator should be presented with an error message inside the ACP. The correct active index can then be selected and the user will not see this error.