The 38-year-old former defender was hospitalised last month and has now entered a crucial phase of his recovery.
Tjarda Zeggelink of Dutch club FC de Rebellen has confirmed that the Drenthe is pushing his limits to recover and get back to full fitness.
"He's trying incredibly hard, works incredibly hard, and shows improvement every day," said Zeggelink.
"What he used to do on the field, he now does off the field as well. Just a true fighter: no nonsense, just action. Royston amazes us every time. We are very hopeful for a good recovery.
In an interview with BBC Sport, he added: "Royston is currently in an important phase of his rehabilitation process. He is working with great dedication and discipline on his recovery, showing the same commitment and motivation, we have come to expect from him.
"While it is still too early to provide a specific timeline, we are pleased with Royston's approach to his recovery.
“His positive attitude and professional mindset are playing an important role in the overall progress of the rehabilitation process. We will continue to monitor his development closely and remain confident in a successful recovery."