SCOTTSDALE, Ariz. -- The D-backs will almost certainly be without their best starter from a year ago when the regular season begins after right-hander Zac Gallen was diagnosed Tuesday with a hairline stress fracture of his right lateral forearm at the radial head.

Before the D-backs' 3-0 loss to the Mariners on Tuesday night, manager Torey Lovullo said the club was not sure yet how long Gallen would be out of action because there is no precedent in baseball for this type of injury. Stress fractures, though, typically take weeks rather than days to heal and Opening Day is nine days away.

"We're just going to get through this first phase, where he's playing catch, and then reload, and I'll keep you guys updated," said Lovullo.

Gallen had an MRI, a CT scan and an X-ray and had those films read by two different doctors to confirm the diagnosis.

For now, Gallen will be allowed to play catch at a low stress level, so he will not completely be shut down from throwing.

Gallen said he first started feeling the discomfort when he was jammed by a pitch while taking batting practice off a pitching machine on March 10.

The right-hander didn't think much of it and went ahead and made his scheduled start March 12. The only time he felt discomfort when pitching was when he threw his curveball. Gallen started getting treatment the following day and he made his scheduled start March 17 against the Rangers.

Forearm discomfort can be a precursor to something more serious such as ligament or tendon issues. Gallen's tests did not suggest any of those issues, which is no doubt a relief to the organization.

"To me, that was the best news possible," Lovullo said. "When we were considering what some of the other options are, obviously, when you [are talking about] an elbow or shoulder and you’re a starting pitcher."

Gallen looked to be slotted in the No. 2 spot in the rotation behind Madison Bumgarner and his absence, however long or short it is, will certainly be felt.

In 12 starts last year, Gallen was 3-2 with a 2.75 ERA, a strong showing that earned him a ninth-place finish in National League Cy Young voting.

Given what Lovullo has heard from Gallen as well as the nature of the injury, it seems likely the injury occurred during a batting practice session in the cage. That it happened while swinging a bat and not pitching is unfortunate, but unavoidable given that the D-backs have to prepare their pitchers to hit in 2021.

"We've tried to perfect our hitting progression with the pitchers," Lovullo said. "We've got to do it. We've got to get these guys ready to step into [the] box and face Major League pitching or otherwise it's going to happen there. But this was the direct result -- according to what Zac has given me, and I believe him 100 percent -- it was consecutive swings where there was not great contact, and he felt a little discomfort."

Lovullo has said repeatedly that he supports whatever rules Major League Baseball puts out regarding the use of the designated hitter.

"I love the strategy of the National League," he said. "However, I’m not thrilled about having one of our main guys go down and get injured."