Replace Default Post Excerpt to Yoast using Filter

functions.php

add_filter( 'get_the_excerpt', 'replace_post_excerpt_crowwwn' );
function replace_post_excerpt_crowwwn($output)
{
    $output=get_post_meta(get_the_ID(), '_yoast_wpseo_metadesc', true);
    if ($output == !NULL) {
		return $output;
	}
	else {
		$content = get_the_content();
		return wp_trim_words( $content , '27' );
	}
}