Bones season 11
'Bones' returns with season 11 on Fox on 1 October, 2015.Fox

"Bones" actor David Boreanaz is reportedly recovering from a "mystery illness" that has affected his work on the hit television series.

According to a National Enquirer report, producers of "Bones" had to improvise in order to hide Boreanaz's fragile frame, reported Radar Online, noting that the actor returned after the summer hiatus looking weak.

This prompted the writers to rework on the script to limit Boreanaz's appearance so that he could recuperate.

"David said he was recovering from jaundice," a source shared. "He's back at work," added an insider on set, "but scenes had to be reworked or cut. His workload was drastically shortened."

However, a TVLine report noted that Boreanaz's illness was because he suffered a bad reaction to an antibiotic, and this delayed his return to the crime drama. This forced "Bones" producers to create a storyline where Booth goes missing in the first episode of season 11.

"Nothing matters to me more than Bones, so I was thrilled when the producers came up with a way for me to take a few extra weeks to recover from a personal health issue without delaying production," Boreanaz said in a statement to TVLine on 24 August. "I'm happy to say that I'm now feeling great and will be back to work tomorrow, and that our 11th season premiere will broadcast on schedule – with me in it."

The season 10 finale of "Bones" saw Booth and Brennan deciding to leave the Jeffersonian behind to start a new life elsewhere. The first episode of season 11 is entitled "The Loyalty in the Lie" and the story will pick up six months after the events of season 10 finale.

According to the synopsis of the episode, "a case with close personal ties to the couple forces Brennan to become involved in the forensic analysis of the remains, and Booth mysteriously goes missing, prompting an FBI investigation into his whereabouts led by Agent Grace Miller (guest star Kim Raver), who works closely with FBI Agent James Aubrey on the case."