Unfortunately, it seems "probable" really means just that—probable.
According to NFL Network's Andrew Siciliano, Jennings became a surprise inactive due to the injury before kickoff on Sunday. Specifically, Chris Tomasson of the St. Paul Pioneer Press reported the move was due to an Achilles strain:
The Achilles tendon plays a crucial role in plantarflexion—or pointing the toes. It does so by connecting the calf muscles—the gastrocnemius and soleus—to the back of the heel.
When the gastrocnemius and soleus contract, they pull upward on the Achilles tendon, which then pulls upward on the heel. As a result, the toes move downward.
Practically speaking, plantarflexion allows an athlete to push off the ground with his toes while walking or running.
In Jennings' case, strong calf muscles provide for a quick start off the line of scrimmage by pushing his toes into the ground. They also let him push off the turf to leap into the air.
Regrettably, just as any tendon or ligament in the body, the Achilles can stretch or tear. It can also become inflamed due to overuse.
In the worst-case scenario, an already weak or inflamed tendon pulls too suddenly on the back of the heel—either due to a normal football move or when another hit bends already-pointed toes back upward—and it ruptures.
Ruptured Achilles tendons almost always require surgical reattachment and several months of rehab.
Fortunately, it seems Jennings' injury is much less severe. Tomasson later tweeted he expects to play next week:
More than likely, the injury constitutes only a very minor strain or inflammation.
That said, even a mild injury can necessitate rest, as injured tissues are weaker tissues. An inflamed Achilles is less able to resist injury-causing moves and stresses when compared to a healthy tendon.
In other words, playing on a significantly injured Achilles carries a relatively large degree of risk.
Should Jennings return to the field too quickly, his injury could worsen—up to and including the aforementioned complete tear—and it may have a higher chance of doing so.
Tomasson clarified that on Sunday morning, pain served as a warning:
Though mild inflammation usually requires only week-to-week evaluation before playing, continued pain implies ongoing healing and an Achilles that is less than 100 percent.
Should the Vikings medical staff have predicted this?
Well, not necessarily. Every injury and healing timeline are unique.
Furthermore, according to Dr. Matthew Matava—professor of Orthopedic Surgery and Sports Medicine at Washington University in St. Louis, president of the NFL Physicians Society and head team physician for the Rams—even the "probable" designation represents only an educated guess.
"'Probable' means (a player) has a 75 percent chance of playing. 'Questionable' means he has a 50 percent chance of playing, and 'doubtful' means he has a 25 percent chance of playing," Matava explained. "It's a crude way to classify the players."
How does a team come to a final designation? Matava continued:
"I can't give you any scientific basis. It's based on experience. It's based on the nature of their injury and how they've responded to rehabilitation. It's not a hard and fast science. [...] It's a way that we can communicate among each other to tell where a guy is in terms of his ability of being able to play in that game."
As the next week rolls on, Jennings may show up on the injury report once again. To gauge his progress, keep an eye on his designation throughout the week.
However, also watch for new media reports all the way up to game time next Sunday. A lingering injury or setback—or an injury that is actually worse than reports let on—is always possible.
After all, if this week showed anything, it's that nothing is a guarantee until a team submits the final "active" or "inactive" decision 90 minutes before kickoff.
Dr. Dave Siebert is a resident physician at the University of Washington and medical analyst at Bleacher Report. Find more of his written work at the Under the Knife blog.
All quotes were obtained firsthand.