"Publisher" compatibility with "Featured Image From URL"

functions.php

<?php

add_filter( 'wp_get_attachment_image_src', 'fifu_replace_attachment_image_src', 10, 2 );

if ( ! function_exists( 'fifu_replace_attachment_image_src' ) ) {
	/**
	 * Replaced attachment src with external thumbnail URL
	 *
	 * @hooked wp_get_attachment_image_src
	 *
	 * @param $image
	 * @param $attachment_id
	 *
	 * @return array
	 */
	function fifu_replace_attachment_image_src( $image, $attachment_id ) {

		if ( $attachment_id == get_post_thumbnail_id( get_the_ID() ) ) {

			$src = get_post_meta( get_the_ID(), 'fifu_image_url', TRUE );

			if ( $src ) {
				return array(
					$src,
					0,
					0,
					FALSE
				);
			}
		}

		return $image;
	}
}