Inbox: Who will fill in for Walker?

Who replaces Taijuan Walker in the rotation?
-- Jordan C., Phoenix
This will become clearer as we get closer to Friday when that starter is needed, but I would think it would be either Matt Koch or Braden Shipley. Here's something else to keep in mind. Whoever starts in Walker's place Friday against the Padres may not stick around after that game or be the one who makes the next Walker start.
When Shelby Miller went down last year, both Shipley and Zack Godley made starts in Miller's place before the team decided to stick with Godley. Following Friday's start, the D-backs could send down whoever it is who starts the game and bring up an extra arm for the bullpen until Walker's turn comes around again.
Also, it's important to keep in mind that we don't know yet how serious Walker's injury is. If it's just a little forearm tightness, he could be back after 10 days. We should know more when we hear the MRI results on Tuesday.
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I've been glancing over and trying to dig deep to find any new, relevant information about Miller, but have come up empty or unenthused by the news I receive. My concern with Miller is the value the team will get for him, in regard to how much it gave up to get him. But even if he's marginal this year and possibly in the future, where do you see him fitting in -- rotation, bullpen or Minors? 
-- Garvey B., Phoenix
I talked to Miller before we left on this last road trip, and he was throwing his bullpen sessions and doing well. They had been looking at a late June or July return for him, but we'll have to see how he continues to progress. It's tough to project that far in advance and say what his role is going to be when he comes back.
If you had asked me two days ago, I would have said there are no open spaces in the rotation. Then Walker has tightness in his forearm, and suddenly, they need a starter. Walker's injury may not be serious and he may be back long before Miller is ready to return, or there may be another hole in the rotation or 'pen at that point. Miller looked very good last spring and at times before being hurt last year, and I think there's a lot of confidence in the organization that when he returns he will be far better than he was in 2016.
Big fan of your articles. My question is sort of a two-parter. After seeing where the team is at so far this season, do you see any prospects in particular who could make a significant impact later this season (kind of like Paul Goldschmidt did in 2011)? Maybe it's too early in their careers, but maybe if they continue to stay hot, catching prospects Daulton Varsho or Andy Yerzy could be an upgrade over Alex Avila?
-- Stephen P., Charleston, S.C.
Thanks for the compliment, Stephen. As for the prospects, given the way the big league roster looks right now, I would think if you're looking for significant impact prospects they would probably come from the pitching side. Whether that's a Taylor Clarke, Shipley, Jon Duplantier, etc. or someone in the bullpen that would be my guess. Reliever Yoan Lopez has been electric early this year for Double-A Jackson, and he has really come a long way over the past two seasons. Varsho is off to a great start in the California League and the organization really likes him, but it would be a stretch to project him up here this year.