It shouldn't be all that shocking that Chad Billingsley is going to the DL, though it should be troubling. After all, Billingsley has had some arm problems in recent years, though they were mostly with his elbow, and not his shoulder. His strained shoulder has no timetable at this time, so there's no telling when he comes back.
The most likely option to replace Billingsley in the rotation is likely to be Severino Gonzalez. He's on the 40 man roster, has already pitched two games this year for the Phillies, and had a very good start on Friday for the IronPigs. He would be on regular rest Wednesday night as well. Other AAA options, including Adam Morgan (not on regular rest, coming off of a so-so performance), Joely Rodriguez (not throwing well, short rest), David Buchanan (pitched Sunday and got hurt), and Phillippe Aumont (would be thrown off schedule and is still as out of options as he was in Spring Training) all have their downside. So Severino Gonzalez would appear to be the favorite.
There is another pitcher in the upper levels of the organization who is on rest for Wednesday though, and that is Aaron Nola. Nola threw Friday, and was his normal, dominating self. He has a 1.77 ERA on the season, and the organization's top pitching prospect has many people calling for his promotion straight up to the big club. Clearly with an open 40 man spot, and an injury, Nola being on turn for a Wednesday start will have people calling for his call-up to Philadelphia.
That won't happen though, nor should it. For one, the Phillies will never have him make his MLB debut in Colorado. Number two, it's important to remember that while Nola looks awesome in Reading, it is AA, not the majors. He has been in the organization less than a year at this point, and has probably not yet progressed into a fully-ready MLB pitcher. His development process coming out of college is probably far ahead of most arms in the system, but it's probably not complete yet. Just as it was probably the right move to start him out in AA this season, it is probably right to not promote him to the big leagues- yet.
Obviously though, if Gonzalez is bound for Colorado to start in Billingsley's place, someone has to start in Gonzalez's place in AAA. Paul Clemens could come off the DL and take that start there, or the newly promoted spot starter Anthony Velasquez could take that turn too, but neither figures as prominently into the Phillies plans as Nola. With Nola dominating AA hitters the way he has, the time has come to challenge him with more polished opposition in AAA. With around 45 innings of AA ball in this year already, he could pitch another 45 innings by the All-Star break, and be ready to join the Phillies after someone is presumably moved at the trade deadline.
Nola is the best bet of anyone to be in the 2016 Phillies rotation. In fact, he might be the best bet of anyone to be in the rotation that ends the 2015 season in Philadelphia. It might sound nice to move him directly to the big league club, but he's probably not ready for that, and the goal here is the long-term. If the long-term goal is Philadelphia, the short-term goal should be Allentown. Given that the Phillies could be filling in for a promoted Gonzalez and an injured Buchanan in AAA, there's really no good reason to not promote Nola this week.