Comments by Horan last night seemed sensible - that McKenzie wants to have a look at each of his 9s during the series. In that regard, it was probably easier/better for McKenzie to give Phipps a shot in the first test and leave out Genia, rather than starting Genia on the bench in the first and then subsequently dropping him (which might not be warranted should Genia show some form off the bench).
To be honest, Phipps's good form aside, I'd feel a lot more confident if we had Genia coming off the bench tomorrow.