Ryan Wood

USA TODAY NETWORK-Wisconsin

GREEN BAY - It’s more than the sheer volume of Green Bay Packers cornerback Sam Shields’ concussion history that could threaten his career.

Shields wasn’t “blown up” in a high-speed, open-field collision Sunday. His helmet didn’t slam against the field. There was no blindside hit. No unexpected whiplash.

The fifth documented concussion of Shields’ football career came on a hard but routine tackle. His head collided against Jacksonville running back T.J. Yeldon’s shoulder, a softer blow than his helmet.

There is no “magic number” for football players to know when it’s time to walk away, Dr. Vernon Williams of the Kerlan-Jobe Center for Sports Neurology told USA TODAY NETWORK-Wisconsin. More troubling, Williams said, is a pattern that develops with each subsequent brain injury.

“If we see a pattern where each concussion takes longer to get better,” Williams said, “the symptoms are more severe, or if we see a pattern where a person is more and more easily concussed. Like, if the first time it takes a big blow, an 80-G impact, a kickoff return where a person gets blown up and everybody in the stadium notices it. And then before long, there’s just kind of a routine hit, or a routine fall to the ground, and the person is concussed.

“If we see a pattern where it’s happening more and more easily, that’s concerning.”

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Williams, a consulting team physician with the Los Angeles Rams, said he “absolutely” has advised professional athletes to end their career because of concussions.

There’s no way to know Shields’ thinking after Sunday's concussion, though retirement would be a tough decision. At age 28, he is in the third season of a four-year, $39 million contract. He would keep his salary this season if he retired because of injury, but pass on $9 million owed next fall.

Regardless, Shields has an extensive concussion history that might force him to consider his future in football. With evidence linking concussions — and even subconcussive blows — to chronic traumatic encephalopathy in retired NFL players, the seventh-year cornerback faces tough questions about his long-term health.

Dr. Andrew Russman, a neurologist at the Cleveland Clinic’s Cerebrovascular Center, said he’s willing to weigh those questions with any athlete who experienced multiple concussions.

“It does happen,” Russman said, “but I think it should be concerning. We should be concerned about people with multiple concussions. We should be concerned because there’s worry about the long-term issues. So I think it’s not unreasonable to be concerned. It seems strange not to be worried or concerned about it.”

The severity of Shields’ concussions have varied. In 2014, he returned from the protocol without missing a game. After smacking his head against Lambeau Field’s frozen tundra last December against the Dallas Cowboys, Shields missed more than a month.

A select few know the severity of Shields’ latest concussion, and they won’t discuss it. Shields, who trotted off the field without assistance but did not return Sunday, is off limits to the media while in the protocol. His agent, Drew Rosenhaus, declined to return an email Friday morning.

It’s hard to imagine Shields returning before the Packers’ bye week at the end of this month. He was listed as doubtful for this week’s game in Minnesota on Friday’s injury report.

“Any time any of our players is in the protocol and dealing with a concussion, it’s a concern,” coach Mike McCarthy said this week. “That’s why we’ll evaluate Sam each and every day, and most importantly to make sure he’s healthy and goes through the challenges of what he’s dealing with.

“We definitely are aware of his history, and that will factor in on how we move forward.”

Each brain injury is an island to itself, Russman said. A fifth concussion’s symptoms can be less severe than the fourth. A player is at higher risk of a concussion in the week to 10 days following injury, which is why NFL teams are expected to use caution before returning to the field.

Because Shields’ most recent concussion came within a year of the one before, Russman said, his symptoms could be worse. Russman said the volume of Shields’ concussion history makes him more at risk to have further concussions in the future.

“Having more concussions is associated with having a risk of future concussions,” Russman said. “It’s associated with having more symptoms if you have a future concussion. So in that way, you can think of it like prior concussion synthesizes us to future concussion, and it makes it more likely to happen again, and it makes it more likely for the symptoms to be more bothersome.”

Russman said Shields’ concussions don’t prevent him from playing this season. The Packers certainly intend on their top cornerback returning to the field. Their secondary would suffer a massive loss without him.

So the Packers will wait and see. They will consider Shields’ concussion history. They will hope he can return to the field, eventually.

There is no hurry, but the threat to Shields’ career becomes more ominous as time passes. If not this concussion, Shields is statistically at greater risk of suffering another, more severe injury to his brain. At some point, he might have to decide whether it’s time to walk away.

“It doesn’t matter how tough you are,” Williams said, “if you’re the starter or second- or third-stringer, or if this is the first quarter or fourth quarter of the Super Bowl, or a preseason practice. Your brain doesn’t know. So when those injuries occur, we have to respect all of them. Every single one of them is important and serious.”

rwood@gannett.com and follow him on Twitter @ByRyanWood