VGFacts

Ninja Gaiden Shadow ("Shadow Warriors" in Europe and Australia) for the Gameboy was not originally planned to be a Ninja Gaiden game. The game was actually going to be a port of the NES game "Shadow of the Ninja" developed and published by Natsume, but Tecmo bought the rights and was reworked to be a Ninja Gaiden prequel.

In Nintendo Power vol. 28 page 67 there is a screenshot of the game where it is still named "Shadow of the Ninja" with the text describing:

Quote:Another one to look for is Shadow of the Ninja, an excellent Game Boy adaptation of the NES ninja thriller. This version includes new settings and new enemies along with the same great play control and the same cool acrobatic moves.

(CLICK IMAGE TO ENLARGE. LOCATED IN THE COMING SOON SECTION)

Source:Youtube-Nintendo Power Volume 28 (Sep 1991) Overview

Here's a screenshot that mimics the same one on that page:



Some of Shadow of the Ninja's music tracks and sound effects were still in the final product:

Final Boss theme
-Title screen Fx
-Landing Fx
-Grabing Fx


Additional Sources:
Internet Archive
1up Review