However, the move appears to be based more so on circumstance than merit.
Here's the reaction from SB Nation's Pirates blog, Bucs Dugout:
d'Arnaud hit well in the second half after a poor first half, but this isn't a promotion based on merit. Yamaico Navarro still isn't eligible to be promoted, because it's been less than 10 days since the Bucs sent him down, so d'Arnaud is next in line. The fact that he's on the 40-man roster already also helps. We'll see what the Pirates end up doing when Erik Bedard's roster spot comes up.
In 97 games with Indianapolis this season, d'Arnaud is hitting .247 with six home runs, 24 doubles and 37 RBI.