According to said sources, the nerves are not healing as quickly as Manning and the Colts had hoped. The vertebrae in Manning's neck have healed, but since starting to throw in December, the velocity in his passes has not improved and the two sources are afraid that it will never return.
More from Stampede Blue: Peyton Manning still isn't healthy.
Two league-affiliated doctors familiar with spinal fusion surgery told Yahoo! Sports that it could take a year before Manning knows if he can return to action. Manning underwent his latest surgery in September 2011.
Manning could not be reached for comment, but the report cited a doctor familiar with Manning's surgery:
"If you're getting consistent improvement, then that's OK. Even if it's going from lifting 10 pounds to 15 pounds to 20 pounds over a stretch of weeks, that's fine," said a doctor who has not seen Manning but has a background in spinal surgery. "If you hit a plateau, that's a problem. ... Now, I say that, but I also tell patients who have been through it that it can take up to a year to find out exactly how much strength you're going to get back.
"Right now, Peyton is at about six months. He should have a much better idea by July or August just how far he's going to get ... even then, that's only a part of it. You can tell about 80 percent of how the nerves and the muscles are healing by rehab. What you really have to see is how his arm holds up when he starts to throw. Does he have the same velocity on the 15-yard out? Can he throw the 60-yard pass? Can he throw for 30 minutes before his arm gets tired? Can he throw for an hour? It's a very complicated process."