Was Demoted for no reason

i switched to this local bank (russian) from multinational, american company because they were proposing me a higher position. I accepted offer, and was working here for 4 months.
And recently they told me that they gonna make me a specialist and put other guy instead of me as a manager.( he was a manager of another department) they say “its nothing to worry about, u will have the same pay, etc”.
But i dont want my resume to look like i was demoted, my friend offered me to refer to some unknown start-up, they will pay same money as i make right now. Should i leave right away and join the start up? It's not so much i want to join them, but leaving current company asap