Sellers took leave on his doctor's orders.
Fullerton Police Captain Kevin Hamilton is acting police chief in the interim.
"I'm sure there's lots of speculation as to what the reason is. I don't have that information, at least not right now," said Fullerton Police Sgt. Andrew Goodrich. "There is no timeframe on it [the length of the medical leave]. Captain Hamilton will be the acting chief for the foreseeable future. That's what I have available at this time and we'll deal with the future as it comes to us."
Hamilton has been with the Fullerton Police Department for more than 30 years.
On July 5, six Fullerton officers were investigating reports of someone burglarizing cars at a transit hub.
They tried to search the backpack of Kelly Thomas, a homeless man suffering from schizophrenia. Police have said Thomas resisted arrest and tried to flee. He was allegedly Tasered and beaten while officers arrested him.
Thomas was left badly bruised and swollen in the face. He was hospitalized with severe head and neck injuries. He was taken off life support July 10 by his family. Thomas is the son of Ron Thomas, a former Orange County Sheriff's deputy.
"I think it's time. It's just precursor to disguise what's really happening. He's going to end up resigning," Ron Thomas said.
The Orange County District Attorney's Office is reviewing footage from a surveillance camera that shows the bloody confrontation between Fullerton police and Kelly Thomas.
Orange County Dist. Atty. Tony Rackauckas called the footage a key piece of evidence. It has yet to be released to the public.
"Once you view it, you just can't help but feel that we have a tragedy here and obviously it shouldn't happen," Rackauckas said.
Rackauckas would not elaborate on what the footage shows, except to say the video shows a struggle and that he had not seen any evidence the six officers intended to kill 37-year-old Thomas.
Rackauckas said his office is trying to determine whether officers used excessive force. The investigation is being expedited, he said.
The six officers were placed on administrative leave.
The Fullerton City Council has ordered a special meeting for Friday. They are also expected to vote on a recommendation by the city manager to hire an independent consultant who would look at the policies and procedures of the department.
In a video taken by a bystander with a cellphone, Thomas can be heard screaming out for his father as officers use a Taser on him. In another video from a bus, witnesses describe how officers beat Thomas and how he was left nearly lifeless.
The coroner's office has not determined a cause of death because it is waiting for the results of toxicology tests.
The Associated Press contributed to this report.