function link_words( $text ){
    //array of words and links
    $words_to_replace = array(
    'guanciale' => '<a href="#">guanciale</a>',
    'lattuga' => '<a href="#">lattuga</a>'
    );
    foreach ($words_to_replace as $old_word => $new_word){
        $reg =sprintf('/<a [^>]+>([^<]+)+[%s]+<\/a>/',$old_word);
            if(!preg_match_all($reg, $text)){
                $text = preg_replace("/$old_word/", $new_word, $text, 1);
                echo '<h1>Ho messo il link</h1>';
            }

        return $text;
    }
add_filter( 'the_content', 'link_words' );
add_filter( 'the_excerpt', 'link_words' );