It certainly doesn't come as much of a surprise. After all, the Diamondbacks' bullpen has enough trouble as it is and the team is in need of another starter to replace Bronson Arroyo who went to the 15-day DL. Will Harris was sent back to Reno today, and Mike Bolsinger was called up. Bolsinger will start in tonight's game going against Kyle Lohse and the Brewers. This adjustment to the rotation pushes back Wade Miley a day. Kirk Gibson publicly announced the move on the Burns and Gambo Show.
This move makes sense as Bolsinger has already seen some work for the big club this year and experienced some moderate success. It also prevents the team from having to make any adjustments to the team's 40-man roster. Given the number of off-days between now and the All-Star Game, Bolsinger could see anywhere from 2-4 starts, depending on how the rotation shakes out. If he performs well, it's possible that Bolsinger would be kept on rotation through the off-days. If Bolsinger has some shaky outings, then he could potentially be skipped twice due to off-days.
All of this assumes that Bronson Arroyo remains on track to return once the break for the All-Star Game is concluded on July 17th, with the Diamondbacks having their first post-break game on Friday, July 18th at home versus the Cubs. As there has been no further word on the condition of Arroyo's elbow, such a return would seem to be likely at this point.