21-year-old Mbulu, who joined the Lions from Brentwood Town in 2015, spent the latter half of last season on loan with Vanarama National League side Braintree Town, making just three first-team appearances.
We understand Mbulu, who was released by Millwall at the end of last season, made an appearance during Thursday evening's pre-season victory over Ajax Amsterdam and looks set to train with the rest of the squad as Keates looks to add to his backline following the injury to hot-prospect Kory Roberts, who looks set to miss the whole season after breaking his leg against Stoke City.