He pitched through it -- thought it was even getting better -- and manager Kirk Gibson said they decided on getting an MRI "just to put him at ease."
The results shocked the club: Yet another key pitcher could be headed to season-ending reconstructive elbow surgery.
"It's kind of deflating," Hernandez said Friday.
Added Gibson: "I don't know what to say. It's disappointing."
Hernandez indicated he has a torn ulnar collateral ligament. Three days after All-Star starter Patrick Corbin had season-ending elbow surgery, Hernandez is likely headed to a similar fate after he meets with Dr. James Andrews in Florida for a second opinion Monday.
The surgery could happen next week.
"We had him slotted in the eighth inning," Gibson said. "Would we rather have David there? Of course, but we're going to be without him for sure, it looks like. We're going to have to reconfigure that. Other people have been preparing very hard."
Hernandez said he first felt pain a couple weeks ago. When it didn't improve, he started getting treatment from the team medical staff. He then said he noticed improvement.
"I even threw two days ago and it felt like it was getting better," Hernandez said.
The news of his injury came the same day Corbin, wearing a black sling, returned to the clubhouse to meet with the training staff. He had surgery Tuesday.
"Me and David are really close," Corbin said. "Just terrible news for him."
Hernandez had an off year last season, going 5-6 in 62 appearances with a 4.48 ERA and two saves in eight tries. In 2012, Hernandez was 2-3 with a 2.50 ERA in 72 appearances.
Hernandez allowed two runs on nine hits in 8 1-3 innings in spring training. He didn't pitch in Arizona's season-opening series in Australia, but Gibson was depending on him to set up closer Addison Reed.
"We've got to pick each other up," Gibson said. "It's a new challenge for us. It's something we have to deal with that we didn't think we'd be dealing with a day ago."
Hernandez, meanwhile, is likely facing a year of rehabilitation.
"Corbin and I are going to be hanging out a lot this summer," Hernandez said. "It's not good, but we'll be able to push each other."