Robby Mook told CNN's Jake Tapper it was the fault of Clinton's staff that the press wasn't told for 90 minutes where the former secretary of state had gone when she left a 9/11 commemoration ceremony early after "overheating" and having to be helped into a van after faltering Sunday.
"I wish we'd done that in a shorter amount of time. That's on us -- that's on the staff," Mook told Tapper on "The Lead." "We were trying to make sure she could see her doctor, getting her to Chelsea's apartment, making sure she was OK."
Clinton departed the ceremony at 9:30 a.m. ET and went to daughter Chelsea Clinton's apartment. Later, Clinton returned to her home in Chappaqua, New York, where Mook said she was attended to by her doctor.
A video showed a wobbly Clinton stumbling as she was assisted into her van to depart the ceremony. Asked if she fainted, Mook said: "My understanding is she did not lose consciousness."
After 5 p.m. ET, her campaign released a statement saying Clinton was diagnosed Friday with pneumonia.
"On Friday, she made a decision with her doctors to just power through this," Mook said when Tapper pressed on why the campaign hadn't disclosed that diagnosis at the time.
"Obviously pneumonia is more serious than a cold or the flu, but often people continue to go in to work and power through" with pneumonia, Mook said.
"If it was up to her, she would be out there right now, continuing to campaign. But we definitely think it's the right thing for her to have some down time, get over this and get back over there," he said.
In the at-times contentious interview, Mook complained that Republican presidential nominee Donald Trump hasn't disclosed as much about his own health -- a brief doctor's note with no vital information is all his campaign has released -- as Clinton.
"Donald Trump has released nothing for 500 or more days," Mook said. "Let's hold Donald Trump's feet to the fire to disclose anything whatsoever this serious about his health."
Trump said Monday on "Fox and Friends" he'll soon make public the results of a physical exam he had last week.
Clinton's campaign, meanwhile, says she'll be examined this week and the campaign will release the full results.
Clinton tweeted twice about her health Monday evening, saying: "Thanks to everyone who's reached out with well wishes! I'm feeling fine and getting better."
Then, in the second tweet, she said: "Like anyone who's ever been home sick from work, I'm just anxious to get back out there. See you on the trail soon."
Both tweets were signed "-H," indicating they came from Clinton herself, not her staff.