"These are tough things for me to address," said D-backs manager Torey Lovullo. "It's really unfortunate. Shelby's been throwing the ball extremely well. Numbers aside, you just hate to have something like his pop up. We're going to be optimistic."
Miller says he has never experienced this type of issue before, but a forearm injury sometimes is a precursor to elbow problems and ultimately Tommy John (ligament-replacement) surgery.
"Personally, I feel like I'll probably feel better tomorrow," Miller said. "I don't think it's that big of an issue. I think it's more just one of those things that flare up when you're out there. Physically, I don't think it's damaging at all. It's just what it is, a little tight.
"Their plan is just to give me an MRI to see where it's at. It's just a safe bet."
Miller faced the first three batters of the fifth inning, walking opposing pitcher Brandon McCarthy and Joc Pederson before allowing a towering Corey Seager two-run double off the center-field wall. Lovullo then brought the D-backs' trainer to the mound, and after a short discussion, removed Miller. He was replaced by Randall Delgado, who allowed three consecutive hits, including a Yasmani Grandal two-run homer.
"It just jumped up on him in the fifth inning," Lovullo said. "Mike Butcher went out for a mound visit. Nothing was said about it. And we just didn't like what we saw. Then I went out there and made that decision to remove him."
Sunday's outing was Miller's fourth start of the season, and he was coming off a 2-1 victory over the Padres at Petco Park.
"I felt it tighten up in the fifth a little and I kind of lost command," Miller said. "I just felt like I couldn't throw the pitches where I wanted them. I didn't have a good feel for it."