How it works
Basically, any player on the 40-man roster is eligible to join the major-league club. There's no compulsion to bring everyone - or, indeed, anyone - up, but it provides a chance for the team to see how young players might perform in the majors. Other factors are taken into account. Is the minor-league affiliate still playing in their postseason? Does the team have need of help at a particular position? Would the player need to be added to the 40-man roster? It's also worth mentioning that players on the 15-day DL do count against the 40-man roster. Players on the 60-day DL e.g. Patrick Corbin, do not.
Our 40-man roster
With the arrival of Nolan Reinhold, the 40-man roster currently sits at 40. That includes 26 men on the active roster (pending a move tomorrow for Reinhold), five on the 15-day disabled list, and nine others. Of the injured players, we can probably expect to see Chris Owings and A.J. Pollock return to the club on or around September 1; Cody Ross's status is uncertain; Paul Goldschmidt is probably not likely to see action (though we did get good news, with no surgery needed on his hand); and Bronson Arroyo won't be going anywhere either.
Let's start off in Triple-A, where the Reno Aces are currently one game behind the Sacramento River Cats in the Pacific Coast League Pacific Northern division. And the schedule makers nailed it, because the two teams start a five-game series against each other, in Sacramento tonight. The Aces don't have much room for error, needing to win four to take the title, with Clayton Richard making his first start for them in the opener. If Reno prevails, they'll play the Las Vegas 51's in the opening round.
In Double-A, the season is divided into two halves, with the winners of the first half playing the second-half champions in the opening round. The Mobile Baybears are already their, having won their division in the first half by three games. As a result, they will play the second-half winners (most likely the Mississippi Braves at this point) in a best of five series, beginning on Thursday September 4, so we may not see any Baybears for a while, depending how that goes.
The Archie Bradley situation
The news that Bradley will be taking part in the Arizona Fall League is another point of data which tends to suggest that he will not be called up this year. The Diamondbacks have said they will possibly go to a six-man rotation down the stretch and it would make sense for the team to want Archie to get his innings count up - due to injury, he has pitched almost 70 innings less than last year. However, it is rare for a player to get a September call-up and then pitch in the AFL as well. He'll probably just pitch for Mobile in their post-season run, and then transfer to the AFL, which starts October 7.
Also weighing against giving Bradley a call-up, is that he isn't currently on the 40-man roster. There isn't a free slot, and we need to find room for the players currently on the 60-day DL, as that doesn't appear to operate over the winter. And it's not even a move that has to be made, because Bradley is not yet eligible to be selected by another team in December's Rule 5 draft. That isn't necessarily an absolute show-stopper - Jake Lamb was added to the 40-man roster when he didn't have to be - the spot may be better used by protecting another prospect from being Uggla'd away by another team.
The nine contenders
A couple of things stand out about the uninjured players currently eligible to come up when the calendar turns over the weekend. Firstly, we've seen almost all of them already. The scope of their participation varies, from the 25 games in which Kieschnick has appeared, down to the single game for Chafin, while Brewer and Paterson only played in previous seasons. But there are no real unknown quantities here. Also, the majority have been starting pitchers in the minors, and one imagines only one of these, or perhaps two at most, will be needed. The others are a reliever (Paterson), an infielder (Ahmed) and two outfielders (Jackson and Kieschnick).
Let's look at the contenders in more detail, with info on their seasons to date, and major-league records.
Minor-league: (AAA) .315/.379/.424
Major-league: .228/.254/.316, 61 PAs
Ahmed made his major-league debut on June 29, when Chris Owings went on the DL, but was deemed surplus to requirements after Cliff Pennington came back and was sent back to Reno on August 8. With Owings also likely to be returning shortly, it's not clear how the team would find much playing time for Ahmed, especially as he is almost a "pure" shortstop, with only 16 pro games elsewhere.
Minor-league: (AAA) 8-3, 3.93 ERA, 88:32 K:BB, 91.2 IP
Major-league: 1-6, 5.50 ERA, 48:17 K:BB, 52.1 IP
Bolsinger had two spells in the rotation, each with five starts: the second (ERA 5.02) went a bit better than the first (6.08), but neither exactly forced his way back into the picture. On the other hand, he has been pretty good in Reno: of the eight pitchers with more than six starts there, Bolsinger is the only one to have an ERA below 4.70. May be in the running for the sixth rotation spot.
Minor-league: (AA/AAA) 11-12, 4.62 ERA, 136:46 K:BB, 163.2 IP
Major-league: (2013) 0-0, 3.00 ERA, 5:2 K:BB, 6 IP
After getting a cup of coffee in 2013, Brewer struggled early, posting a 7.28 ERA in his first nine starts with Reno, being demoted to Double-A as a result. He seemed to get things back on track there, and has been much improved since returning to Reno at the beginning of July, with a 3.93 ERA and 62:12 K:BB ratio over those 12 starts, totaling 73.1 innings of work.
Minor-league: (AA/AAA) 8-7, 4.04 ERA, 110:53 K:BB, 142.2 IP
Major-league: 0-0, 0.00 ERA, 3:2 K:BB, 5.0 IP
Chafin was lights-out in Mobile at the beginning of 2014, with a sub-two ERA (1.96) in nine starts, but that didn't quite translate to Reno. He had a 5.40 ERA there when called up for a spot-start in the night-cap of the double-header in Cleveland, and threw five shutout innings. His two starts back in Triple-A have been back to mediocre (11 IP, 6 ER), but still might be the frontrunner for the sixth rotation spot.
Minor-league: (AAA) .208/.299/.350, 271 PAs
Major-league: (2012, CHC) .175/.303/.342, 142 PAs
The only one of the nine not to have played for the Diamondbacks at some point, Jackson was acquired from the Cubs on August 14 for minor-league reliever Blake Cooper, after a waiver claim by Arizona. Jackson was a 1st-round pick in 2009, and ranked in the mid-30's of all prospects before 2012, but a ferocious K-rate caused his stock to plummet. He has whiffed in 38% of his PAs this season.
Minor-league: (AAA) .267/.322/.466, 382 PAs
Major-league: .195/.195/.293, 41 PAs
We saw what he could do at the major-league level and, to be honest, it wasn't much. A .788 OPS at Reno doesn't inspire confidence either - for comparison, it's basically what Tuffy Gosewisch had there last year (albeit pre-humidor). The return of Pollock, and potentially Ross, likely means our outfield will be full, given Mark Trumbo, Ender Inciarte, David Peralta and Alfredo Marte as well.
Minor-league: (AAA) 0-2, 3.18 ERA, 29:18 K:BB, 39.2 IP
Major-league: (2013) 0-0, 3.86 ERA, 2:0 K:BB, 2.1 IP
The apparent invisibility of Paterson remains a mystery. He had a great rookie campaign in 2011, but imploded totally at the start of 2012, and rarely been seen since, despite an ERA in Triple-A over the past two years of 2.74. Largely replaced as a backup leftie by Eury de la Rosa, even the trade of Joe Thatcher doesn't offer much hope, as Matt Reynolds should be back in 2015. Likely to remain invisible.
Minor-league: (AAA) 9-8, 6.34 ERA, 79:46 K:BB, 130.2 IP
Major-league: 0-1, 7.88 ERA, 5:1 K:BB
Schultz finally made his major-league debut as a 28-year-old in Sydney, the sixth-oldest in Diamondbacks history. It only lasted one inning, but he was recalled at the end of July, to help out the bullpen. He took the loss in his second game there, after allowing five earned runs in two-thirds of an inning, and was sent back down after one more mop-up appearance. May have a similar role in September, but peripherals suck.
Minor-leage: (Rk/AAA) 3-8, 6.22 ERA, 74:19 K:BB, 81.0 IP
Major-league: 0-1, 4.82 ERA, 7:3 K:BB, 9.1 IP
Got a spot-start in May for a double-header and a long-relief out in June, but also missed some time in July, hence the rehab assignment in the AZL. Still kinda fringey, but I note his minor-league K-rate was significantly better this year at 8.2, compared to just 4.9 in 2013. His walk rate was better too, but he has allowed more hits and more home-ruins, leading to that bloated ERA.
If I'd to call it, I would say Chafin is the most likely to become the sixth starter, with perhaps Brewer or Bolsinger in the running, if the team decides Chase Anderson needs to hit the showers for the season. Ahmed may also get the benefit of a call-up, as he's the most obviously "prospect" among these nine names, though finding playing time for him will be an interesting exercise. I don't see any of them as moving the needle much - and that's probably okay, since this is a team less in need of moving the needle, and more seeking an entire replacement meter.