Report: Peyton had fourth neck surgery
Manning, who will turn 36 next month, was known to have had three neck surgeries in a 19-month span, with the most serious operation occurring in early September to fuse together two vertebrae.
Citing NFL sources, SI.com reported Wednesday that another procedure occurred last summer, while the NFL lockout was in place and teams had little contact with their players.
In May 2011, Manning underwent surgery to correct a bulging disc, then reportedly needed a follow-up procedure that was not made public. His first neck surgery was during the 2010 offseason.
The report states Manning has developed bone spurs above the area where the spinal fusion was performed Sept. 9, leading the Colts to believe he will eventually need more surgery.
Manning's agent, Tom Condon, refused to comment to SI.com.
Doctors have ruled that Manning's neck is structurally sound enough for him to play football again. But the lingering question is at what pace he will regain his arm strength, since nerves in his neck must still regenerate.
The four-time MVP has said he expects to be healthy enough to play next season after missing all of 2011 and watching the Colts finish a league-worst 2-14.
The uncertainty around Manning's health has led many to conclude the Colts will cut ties with him, and instead select former Stanford quarterback Andrew Luck with the top pick in April's NFL Draft.
The Colts owe Manning a $28 million bonus if he is on the roster March 8, making it unlikely the team can afford both quarterbacks unless Manning agrees to a substantial pay cut.
On Tuesday, Colts owner Jim Irsay maintained he wants Manning to return to the Colts if a new contract can be worked out. The two are expected to meet soon to discuss Manning's future with the team.