Just before the end of August, O's made a couple of trades to improve the depth of the roster. While Kelly Johnson can certainly serve as a valuable left-handed pinch hitter (he might be the best available option), Alejandro De Aza has the potential to make a larger impact on the O's as a possible starter. Why should De Aza be given the start against righties over Delmon Young?
First, let's address the offensive side. While the Young is probably the superior hitter between the two, the gap against righties is a lot less than one might imagine. Over the most recent three seasons, De Aza and Young have had an identical wRC+ of 97. While De Aza's batting performance has been trending down, Young's has been trending up. However, against right-handed pitchers, over the same three seasons, De Aza has outhit Young with a wRC+ of 102 compared to 92. Even in a down season in 2014, De Aza has hit righties at a clip of 108 wRC+ (Young has had a 133 wRC+ against righties this season). There might not be a gap between the batting performances of the two against righties at all. Even if there is, the difference is minimal.
Next, there is defense to consider. The optimal lineup including both Young and Nelson Cruz probably has Cruz in LF, since I am not even sure Young is able to catch a fly ball that goes directly toward him at this point. It is undeniable that De Aza is a better fielder than both Young and Cruz. The real question is the degree to which he is better. For the season, Cruz has been a perfectly average LF by both DRS and UZR. On the other hand, De Aza has been about 3 runs above average in LF by both DRS and UZR in approximately 100 games. For the past three seasons, Cruz was consistently below average in RF by DRS and UZR, so the most optimistic outlook for Cruz as a LF is a slightly below-average one. De Aza, meanwhile, has been a regular CF before this season. While both DRS and UZR rated him as a negative CF (in the case of DRS, extremely negative), there is such a huge gap between CF and LF in terms of defensive ability that De Aza would still rate positively in LF. Overall, the difference between De Aza and Cruz in the field is about five runs over a full season.
De Aza is a clear upgrade over Young in the starting lineup against righties, who start more than 60% of the games. There is no reason Young should start a single game in the field against a right-handed pitcher as long as De Aza is healthy. To be able to achieve a real upgrade at the end of August without giving up any piece of real value is a huge coup for the O's, especially considering that De Aza is still under team control for next season. I am not one who praises the work of Dan Duquette and the front office frequently, but this is one trade that I have to applaud the front office for.