Indianapolis Colts' quarterback Peyton Manning is struggling with a slow recovery after his neck surgery in September and facing questions about whether he will end his football career . But doctors say a slow recovery is part of the playbook for the type of surgery Manning had.

In September, the four-time MVP had a "single level anterior fusion" procedure, his third operation in 19 months. In the surgery, doctors remove a herniated disc from the spine to alleviate pressure that comes when the disc pushes on spinal nerves. The bulging disc can be painful and can cause weakness in the arms and shoulders.

Once the problem disc has been removed, the bones of the spine should fuse and the nerves should recover. But the process is slow. In December, ESPN reported that the bones of Manning's spine had successfully fused. But athletes usually face three to six months of rehabilitation to recover nerve and muscle function, or even longer if they've had more extensive nerve damage.

"It's a waiting game. Nerves grow at about an inch a month, and you have to wait until the muscle is re-innervated and regains its strength," said Dr. Gerard Varlotta, an associate professor of orthopedics and rehabilitation medicine at New York University Langone Medical Center. "It's like watching grass grow."

Varlotta, who treats professional athletes with injuries similar to Manning's, said while the nerves are re-growing, the muscle they're trying to reach may become atrophied, making a quarterback like Manning struggle to throw strong passes or grip the ball.

"If there is identified nerve damage, there may be slower progression of return of strength and motor function," Varlotta said. "For a professional athlete who's 35 years old, this is a very concerning injury."

Dr. Mark Knaub, an orthopedic spinal surgeon at Penn State Hershey Medical Center, said it's not uncommon for a full recovery to take up to a year, especially for professional athletes who require optimal strength.

"It's impossible to know going into it how long it will take. The reality is some people don't recover it all," Knaub said. "People may want to hear some definitive decision about whether or not he'll be able to play, but it may just be too early to tell."

Eyes in the sports world will be on Manning's recovery and future with the Colts this weekend as his brother Eli, quarterback for the New York Giants, travels to the Colts' home stadium to take on the New England Patriots in the Super Bowl.