He re-joined Schalke on loan in January but managed just one appearance on the final day of the season.
Rahman is now back up to full fitness and is eager to make up for lost time.
“Never too late to become the best in the world," he wrote on Twitter.
Schalke have endured a difficult start to the Bundesliga campaign, losing both of their games.
Rahmen added: “It has not been the best of starts to the season but working every minute to turn things around quickly."