Earlier quoted context omitted.
NATO is irrelevant here. A strict interpretation of the Budapest Memorandum, however, has Russia in violation, which would oblige the US and UK to respond.
Strict interpretation says that the latest legitimate president of UA has been overthrown in coup and asked for help. That means, the currently acting representative of Ukraine are not legitimate and do not represent the Ukraine the Budapest Memorandum was about.
Not to precipitate a debate over semantics, but I don't believe being ousted by your own parliament falls under any of the standard definitions of a coup d'état.