Zac Gallen was supposed to be pitching against his former team Wednesday night, but the D-backs right-hander was scratched from his start against the Marlins and placed on the 10-day injured list with what manager Torey Lovullo said was a "minor sprain of one section of his UCL [ulnar collateral ligament] in his right elbow."

Matt Peacock took the ball in Gallen's place Wednesday night, making his first Major League start.

Here's what we know and don't know about Gallen's status:

How was it discovered?
Gallen felt a little tightness after his seven-inning one-hitter against the Braves on April 25. That's not completely uncommon for a pitcher, so he received some treatment on it and it went away.

Gallen didn't experience any issues in his start against the Rockies on May 1, but he did after facing the Mets on May 7 in New York.

The difference this time was that the discomfort had moved from the forearm farther up closer to the elbow. That's when Gallen informed the training staff, which led to a precautionary MRI that revealed the sprain.

"We went and got the MRI, more so we thought it was for peace of mind, but maybe it caught something that could have turned out to be really serious," Gallen said.

How serious is the injury?
Anytime you're talking about the UCL and a pitcher, it's a cause for concern, but the D-backs feel like they got the best news possible out of the MRI.

"From my understanding it’s a very minor sprain, so that’s a positive," Gallen said. "Anytime you get into that area, especially for a pitcher, the news is never really great. So, I think relatively speaking, a minor sprain is a small win here."

Is he going to need Tommy John surgery?
No one has a crystal ball, but based on what he's been told, Gallen does not think so.

"From my understanding, it sounds like surgery will not be needed," Gallen said "So knock on wood in that sense."

What's the next step?
Gallen will be shut down for two weeks, and during that time the D-backs will consult with the doctors who have looked at the MRI to determine how to proceed. While the medical staff is in agreement on what the injury is, there will likely be some differences in specific rehab steps, so the D-backs will need to sort that out.

"I think that means there’s going to be some period of no throwing on the short end and then we will re-image, take a look at how it’s healing, then go from there," D-backs GM Mike Hazen said. "We’re hopeful that this is on the minor end of this injury, but we’ll see how that goes, depending on what everybody says and how he’s feeling."

What will the D-backs do with Gallen's spot in the rotation?
Right-hander Taylor Widener is progressing in his rehab from a strained right groin strain and could rejoin the rotation in the next 10 days or so.

Peacock got the nod for the emergency start Wednesday, and he is a possibility in the future. Right-hander Seth Frankoff was added to the roster Wednesday and is another possibility, as he has a starting background.

Then there's a host of Minor League candidates, including No. 7 prospect Corbin Martin, No. 20 prospect Josh Green and No. 22 prospect Jon Duplantier.

The D-backs are also carrying 14 pitchers, that means they have nine relievers. As a result, Hazen said it is possible they could use a bullpen game at some point to cover the spot.

Any other injury news Wednesday?
Unfortunately for the D-backs, yes.

First baseman Christian Walker, who left Tuesday night's game early, was placed on the 10-day injured list with right oblique soreness. Walker was on the IL with a right oblique strain from April 12-May 3.

Walker said that he doesn't feel the same amount of discomfort he did when he suffered the initial injury and an MRI showed that it was not as serious this time around.

Who replaced Walker and Gallen on the roster?
Infielder/outfielder Andrew Young was recalled from Reno to take Walker's spot, while Frankoff was added to the 40-man roster and called up.

In order to make room on the 40-man for Frankoff, infielder Wyatt Mathisen was designated for assignment.