WWE confirms report from WWE's The Bump that Riddle is injured and will not participate Summerslam. Seth "Freakin" Rollins attacked Riddle at Monday Night Raw resulting Riddle suffering a brachial plexus injury (a stinger). "The injury is the network of nerves that sends signals from the spinal cord to the shoulder, arm, and hand. A brachial plexus injury occurs when these nerves are stretched, compressed, or in the most serious cases, ripped apart or torn away from the spinal cord" - report by Mayo Clinic.
As described by WWE, signs for this injury are weakness in the hand or arm. We can assume that Riddle's injury was caused by the curb stomp by Rollins. A less severe injury could take a few seconds or minutes to recover. However, due to the postponement, it may be severe and may require surgery to restore function.
For Seth Rollins, this is the second superstar that suffered an injury. Cody Rhodes defeated Rollins three times. Before the third victory, Rhodes torn his pectoral tendon completely off the bone before his match at Hell in a Cell. He wrestled through the injury and won the match.
WWE announced that any additional updates in Riddle will be available in the WWE website.