The thing is, this is the best transaction Mr. Duquette made out of all his mid-season acquisitions. At the time of the deal;
1. Jake Arrieta- a 27 year old who couldn't throw strikes, couldn't retire AAA hitters effectively, and who never improved throughout his four years with the Orioles. In fact, he got worse every year he was in the majors. It was all but assured he was never going to succeed in Baltimore. He was no longer a prospect, he was simply a "had-been/bust" for the Orioles, so it was time to stop treating him like one.
2. Pedro Strop- a 28 year old reliever who couldn't throw strikes, couldn't get major league hitters out period for the last year, and was out of options in the middle of a pennant race.
1. Scott Feldman- an above-average starter Baltimore had control of for half the season. At the end of the season, the team had the increased possibility to re-sign him or the ability to extend him a qualifying offer(if he pitched a little bit better he might have) in order to retain or pick up a draft pick.
2. Steve Clevenger- a 27 year old left handed hitting catcher who can catch and has shown he can hit at AAA. He is under team control for five more years. At this point, he is slated to be the left-handed part of the DH situation. At the very least, he is a serviceable back-up catcher to Matt Wieters
The Orioles were in the middle of a playoff chase, and they needed rotation stabilization. Scott Feldman looked to be the right guy at the time, and turned out to be pretty good for Baltimore in fifteen starts. Mr. Duquette didn't re-sign Feldman, but the trade isn't over. Clevenger will most likely provide positive value over the next few years.
Just because a trade didn't work out, doesn't mean it wasn't the right trade at the right time. The objective of mid-season trades from the buyer's perspective is to acquire talent for the present at the expense of talent for the future(prospects). Mr. Duquette was able to avoid giving up prospects. Most of a prospect's value comes from;
(1) the benefit of multiple cost-controlled years they provide
(2) the benefit of using prospects in leverage in trade
(3) the benefit of saving money on the prospect's position and placing that money elsewhere on the team
Arrieta and Strop don't really apply to these benefits anymore. They are about to start getting expensive to control, and that is if they are even succeeding in the majors in the first place.