The Lord of Darkness
Ok.Yup, they showed a flashback of him asking it for its help for his upcoming battles prior to his match with Sawyer. He also drop mentions that originally he was thinking of calling in one of his Pokémon from Oak's lab.
In that case if Naganadel wouldn't return he would either use his Ride Garchomp or consider one of the Reserves.
Though, everyone believed Poipole will return.
There's always Nebby...