In a season that has seen numerous organizations call up numerous prospects well before they were ready (Mariners, I'm talking to you), the Tigers were extremely patient with not only their top prospect, but their one true impact player in the minor leagues. It is, of course, much easier to be patient when your lineup is stacked with the likes of Miguel Cabrera and Prince Fielder.
That prospect is Nick Castellanos, who has been in the big leagues since the rosters expanded on September 1st. In the Tigers four games since his promotion, the former third baseman and current right fielder has yet to make a start and has received just two plate appearances. It appears he will get a start this weekend, according to Jason Beck of MLB.com, but the question that begs to be asked is, why has it taken this long?
We don't know what kind of hitter Castellanos will be right away, and a pennant race isn't a great place to test out new prospects, but we do know that he has a strong track record of being an above-average hitter, we know he's made significant progress this season in Triple-A, and we know he projects to ultimately be a significantly better hitter than the guys he's currently backing-up. Castellanos improved every aspect of his offensive game this season (while spending the whole year in Triple-A), including his power production and his walk rate.
Torii Hunter isn't going anywhere as the everyday right fielder, so any playing time Castellanos is going to get in the outfield will come in left. The left field platoon for the majority of the season has been between the left-handed hitting Andy Dirks and the right-handed hitting Matt Tuiasosopo, with the term "hitting" being used quite liberally.
Dirks is a solid defender, but even against right-handed pitching this season he has just a .690 OPS. For his career, he's only at .750 against them, and that's primarily fueled by last year's BABIP frenzy. In general, Dirks isn't much of a hitter no matter who he faces.
Neither is Tuiasosopo. Don't be fooled by his .272/.378/.463 line on the season. After an incredibly hot first three months, Tuiasosopo has all but disappeared since the start of July, hitting .211/.286/.368 over the past two-plus months. Additionally, as the right-handed portion of the platoon, he faces mostly left-handed pitchers, but even with his hot hitting early in the season he's hit just .236 against lefties. All he does against lefties is draw a decent amount of walks. Most of his production this season has come due to a fluky reverse-platoon split which has seen him go 15-for-41 against righties.
The Tigers have also had another way of getting Castellanos in the lineup, one which they've failed to take advantage. The reason Castellanos is now an outfielder is the presence of Cabrera at third base. Cabrera, however, is currently hobbled by injury and has started just one of the four games since Castellanos was promoted. Castellanos is not going to play third base in Detroit long-term, but he's certainly capable of filling in there while Cabrera is out as a way to test out his bat, and he's certainly a better hitter than Cabrera's replacements, Don Kelly and Ramon Santiago.
The Tigers have little to lose by seeing just how ready Castellanos is to contribute right now. If he's not ready, they can always go back to the Dirks/Tuiasosopo platoon in left field. It's not going anywhere. Perhaps Castellanos isn't ready to contribute any more than that platoon is just yet, but his minor league track record would suggest that he is. I've never seen Castellanos as the all-star caliber hitter that some have, but he proved this season that he's closer to becoming that player than he was earlier in his minor league career. He has nothing left to prove in the minors and he's on the major league roster just waiting for a chance. At the very least, he won't be a worse hitter than Dirks.
I wish I had a better answer as to why Castellanos isn't playing, but the data at hand seems to point to the fact that he should be.