Now at PSG, Bulka has spoken about an incident where Chalobah fell to the ground in training, and Costa thought he was faking.
The Spanish striker was furious with Chalobah, and they nearly came to blows.
The incident is said to have happened in 2016, before the Blues' League Cup loss to West Ham United.
"One time in training Chalobah was on the ground suffering from a tackle. Diego thought he was acting it up so started talking s*** in Spanish," Bulka told Polish YouTube channel Foot Truck, as translated on reddit.
"On top of his game, he was a player Chelsea needed, he could have been a proper [Didier] Drogba successor."