Source: The Wrestling Observer Newsletter
According to WWE sources, there is talk that Daniel Bryan may need shoulder surgery. Bryan underwent a full medical exam recently and it revealed a lot of different injuries that he has with both his neck and shoulder.
Due to the uncertainty as to whether he’ll need surgery and what surgery it could be, he’s not being booked in any long-term plans as they simply don't know when he'll be back.
The original plan was for Roman Reigns to be the top guy going into WrestleMania 31 whether Bryan was hurt or not. It's obvious that they are still going that route with Reigns' recent appearances on WWE television. There are many people within WWE who see HUGE potential in Reigns and that's the reason you're seeing him being pushed so hard.
With that being said, Bryan would have still been WWE Champion with John Cena being the "top" guy. Reigns' push would have been slowed down as there is still a lot of time between now and WrestleMania 31. The original plan before the Daniel Bryan injury/surgery was for Reigns to work a match with Randy Orton and likely a match with Triple H at SummerSlam. This would have all been coming out of the Evolution vs. Shield storyline.