Score a point for Baseball America, because they did nail the first selection, as Steven mentioned in this morning's preview. We chose 21-year-old Oscar Hernandez, who despite his young age, has already been in the Tampa farm system for five seasons. He spent 2014 at Class A Bowling Green, where he was still 1.4 years younger than the average player in the league, and put up a .249/.301/.401 line, for a .701 OPS overall. He's most well-known for the phenomenal season he had in the Venezuelan Summer League in 2011, batting .402 with 21 home-runs - nobody else in the league hit more than eight, so there is power there.
BA called him an "extremely toolsy catcher with a big arm and equally big power... If he develops, Hernandez could be a catcher who limits running games while hitting 15-20 home runs." However, they noted that he "isn't a fully polished catcher defensively yet," and "projects as a below-average hitter, albeit with above-average power potential." In order for the Diamondbacks to retain him, Hernandez needs to be kept on the 25-man roster for the entire 2015 season: if we can't do that, he must then be offered back to the Rays, for half of the $50,000 cost. Even as a backup catcher, it'll be a stretch for him to go from A-ball to the majors.
History isn't on his side, either. Since the Diamondbacks played their first game, in 1998, the only Rule 5 pick to have made it through the year was left-handed reliever Joe Paterson, who was chosen in December 2010. He then had an excellent rookie campaign, with a 2.91 ERA in 62 games, before apparently falling completely out of favor (6.1 MLB innings in 2012-14) and being released last month. The D-backs' last three choices have all failed to stick on the roster, through 2011's Brett Lorin was retained in a trade. Since then, we've seen Starlin Peralta and Marcus Mateo both returned to the team from which they were picked (coincidentally, the Cubs in both cases).
If he's going to be retained, there will likely need to be a 40-man roster move made to begin with. Arizona's is now full, and with what appears likely to be the trade of Wade Miley to the Red Sox for two players who would need spots, a space will need to be cleared. It has been suggested that this may involve designated this year's backup catcher, Tuffy Gosewisch for assignment. He would probably make it through waivers, and could then be stashed away in Reno, for use if Hernandez proves unable to stick. But the odds are he'll follow the last position player chosen by Arizona - catcher James Skelton in 2008 - and be returned to his former team.