Like Sergio Santos recently, Eno Sarris has been placed on the paternity leave list. In his honor, let’s take a look at three babies on their way to the big leagues – a few prospects who are on the verge of big-league playing time not only due to their own performance, but because of the situation on the big club ahead of them. Everyone knows that Mike Trout & Bryce Harper are coming, so instead we’ll check in on…

Nolan Arenado, 3B Colorado Rockies

In 2011, the Rockies sent out nine different players to man the hot corner, from Ty Wigginton‘s 62 starts all the way down to the one sole inning catcher Chris Iannetta received at the end of a blowout loss against Kansas City. Six of the nine – including the top four, who combined for 119 starts – are no longer with the club, a hole the Rockies attempted to patch over by signing 38-year-old former Dodger Casey Blake this winter. Blake was injured & ineffective in camp this spring and was cut before Opening Day, so the Rockies are limping along with utility infielder Chris Nelson & former catcher Jordan Pacheco at the position for now. Neither is hitting (two extra-base hits and three walks between them in 39 PA thus far) and neither has much of a minor-league track record to suggest that’s likely to change any time soon.

Nelson & Pacheco are just placeholders, however, and short-time ones at that, since the team is just marking time until consensus top prospect Nolan Arenado is ready to claim the spot. Arenado tore up the California League last year as a 20-year-old (.361 wOBA, 20 homers, just 53 strikeouts in 583 PA) and then impressed in the Arizona Fall League. If not for the fact that he’d never played above High-A prior to this season, he might have broken camp with the big club – a promotion many were clamoring for anyway – and so far he’s crushing Double-A, hitting .395/.477/.632 in ten games for Tulsa. While there’s obvious small-sample-size caveats in 44 Double-A plate appearances, the scouting reports and complete history have all been glowing.

Arenado turns 21 today, April 16. He may not get a call to the big leagues for his birthday, but you can bet he’ll be spending most of this year in Colorado, not Oklahoma.

Anthony Rizzo, 1B Chicago Cubs

All spring, we heard that the Cubs were really going to give 29-year-old minor-league vet Bryan LaHair a chance to be the regular first baseman, despite having shipped off flamethrower Andrew Cashner to San Diego to reunite Rizzo with Theo Epstein & company. LaHair suffered through a homer-free spring training while Rizzo impressed, but the Cubs insisted that Rizzo would be starting the year at Triple-A Iowa. He did, but then the Cubs went out and started utilty guy Jeff Baker at first base in the first two games of the season and in four of the first nine, as LaHair dealt with back tightness and sat against lefty pitching.

To his credit, LaHair has produced when he’s played, hitting .348/.423/.696 with two homers in the early going. On the farm, though, Rizzo has been the Matt Kemp of the PCL, already hitting five homers to go with a .520 wOBA so far. (Again, small-sample-size and PCL offense to be taken into account here.) If LaHair keeps hitting, he’ll keep his job, but the Cubs clearly hold Rizzo in high regard and didn’t trade Cashner for him to have him waste away in the minors all season long. It remains to be seen how they would make that work – LaHair does have some outfield experience in the minors, and David DeJesus could potentially shift to center if they trade or dump Marlon Byrd, though an outfield defense with both LaHair & Alfonso Soriano sounds hilariously awful – but with a Cubs offense struggling to get anything going, they’ll find a way to get Rizzo in the lineup if he keeps showing that he has nothing left to prove in Iowa.

Trevor Bauer, SP Arizona Diamondbacks

I’m not sure if there was a bigger regression candidate heading into 2012 than Diamondback starter Josh Collmenter, who was surprisingly effective in 24 starts as a rookie last year but who gave little indication based on his history and performance that this was something he’d be able to keep up over the long term. He was basically the Chris Johnson of the new season, in terms of “there is just no way that guy is going to be as good next year.”

While it’s been only two starts, Collmenter has indeed been atrocious, allowing 14 baserunners, 10 earned runs, and 3 homers in 7 innings so far. It’s been bad enough that Kirk Gibson announcing that Collmenter will get another start this week against Atlanta actually qualifies as news, because it was in no way assured. For a team picked by many to win the division this year and possibly go deep into the playoffs, those results are unacceptable.

Even worse for Collmenter, the Diamondbacks have a ready-made replacement in their own top prospect, 2011 #3 overall pick Trevor Bauer. Bauer reportedly came close to making his big-league debut last year and is now one of several hot pitching prospects in the Arizona system, along with Tyler Skaggs, Pat Corbin, and Archie Bradley. With Jarrod Parker off to Oakland in the Trevor Cahill deal, Bauer is the closest to the bigs, having stuck out 46 in 32.1 Double-A innings between his debut last year and his first outings this year. While he needs to work on his walk rate – 5.6/9 in Double-A is far from ideal – he’s the kind of high-upside strikeout arm that the Diamondbacks currently don’t have in the back of a rotation that features Collmenter and Joe Saunders. Collmenter may have just one more start to prove that he’s actually a starting pitcher worth hanging onto, and the odds aren’t good that he’s going to be able to do it.

Bauer may not be the first choice if Collmenter loses his job – Arizona could try to patch things over for a few weeks with similar placeholders Barry Enright, Zach Kroenke, or Wade Miley – though none of them are going to stand in the way of Bauer once Arizona decides he’s ready. By the looks of things, that may be sooner rather than later.