<?php

function replace_thumbnail( $id, $object_id, $meta_key ) {

    if ( '_thumbnail_id' === $meta_key && 'post' === get_post_type( $object_id ) ) {
        remove_filter( 'get_post_metadata', __FUNCTION__ );

        $default_id_img = 91;

        $id = has_post_thumbnail( $object_id ) ? $id : $default_id_img;

        add_filter( 'get_post_metadata', __FUNCTION__, 10, 4 );
    }

    return $id;
}

add_filter( 'get_post_metadata', 'replace_thumbnail', 10, 3 );