According to my sources backstage, the main event of SummerSlam is expected to be Seth Rollins vs Brock Lesnar vs The Undertaker. Officials are worried about Undertaker's health, as his knees and back are "going through hell", according to doctors, which is why they decided for a triple threat match. Furthermore, the match ended in a no contest because Vince McMahon felt it was best to preserve Brock Lesnar's credibility.
The current plan is for Rollins to retain the title at SummerSlam against Lesnar and Undertaker. Officials, especially Vince, are extremely worried about the star power of the full-time roster, so Rollins going over Undertaker and Lesnar is meant to solidify him as a cornerstone of the company.
As of this writing, Rollins is scheduled to begin his feud with Triple H after SummerSlam