"It's going to take me down for a while but not out for good," said Davis, who spoke at a news conference after facing the Colorado Rockies in a Chase Field exhibition Friday night.
"I know I'm not going through this alone," the 32-year-old Davis said. "I know I've got all the help in the world, and I'm definitely optimistic of the outcome."
Davis made his scheduled start in a 12-4 loss to Colorado on Friday night and struggled, giving up back-to-back-to-back home runs in the second inning to Brad Hawpe, Yorvit Torrealba and Jayson Nix.
Davis left after 2 2-3 innings, allowing eight runs and nine hits, including four home runs. He struck out five.
The Chase Field crowd of 24,663 gave Davis an ovation as he walked off the mound.
Davis did not attribute his poor outing to his diagnosis. "I really didn't think about it at all," Davis said. "Believe it or not, I really thought I was executing pitches, and it just felt like everything got hit out of the park. It was just one of those days, I guess."
Davis said that pitching helps keep his mind off his illness.
"Maybe I'll spend more time more time watching tape and figure out what I did wrong today," Davis said with a chuckle.
Doctors discovered a lump in Davis' throat during a routine physical on Feb. 6, Davis said. On Wednesday, biopsy results revealed it to be cancerous.
Davis will have his thyroid gland removed and is expected to be out of the hospital the day after the operation, Diamondbacks head physician Dr. Michael Lee said.
Lee said the team doesn't have a projected return date for Davis but said it could be within four to six weeks.
"We're extremely optimistic," Lee said. "This is a bump in the road for Doug, but we see him being with us the rest of the season. It's just a matter of when that will happen."
Davis said his mother also had thyroid cancer and made a full recovery.
After talking with doctors, Davis asked to keep pitching until his operation. The club said Davis, the team's No. 3 starter, would make his regular starts on April 3 at Cincinnati and against the Los Angeles Dodgers on April 8.
Davis called manager Bob Melvin with word that he had cancer, then began lobbying to stay in the rotation.