It's a matter of style rather than of grammar. Many native speakers find it completely acceptable. Who was the grammarian, by the way?
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James MA world-famous grammarian says that "really" is unacceptable in that sentence. One must use "completely."That's right. The famous grammarian is correct. It's logical and meaningful to use completely. I would never use really in that sentence. It sounds very odd.
prajwalkr It's logical and meaningful to use completely.I agree that it's logical to use 'completely. However, logic is not, in itself, an infallible test of what is accepted in language.